DANIEL Gilmour had gone to work as normal at the metal coating plant of ICL Plastics. For the first few hours, it was an ordinary day at the plant off Maryhill Road – until the stroke of midday.

“All of a sudden,” Mr Gilmour recalled, “I heard a huge bang, and then the roof fell in on us and everything went black.

“A wee hole in the wall appeared where the explosion had been. I grabbed hold of my mate Jimmy and we just sort of held hands and moved our weight towards the hole in the wall. We managed to crawl out, but the people on the upper floors must have been caught in the rubble.”

Police received initial reports of the explosion at 12:01pm.

Eyewitnesses described seeing the concrete roof of the Victorian brick building rise into the air, before falling back, creating a concertina effect, crushing the four floors below and trapping staff as it fell.

Mr Gilmour and his colleague were working on the ground floor. They were lucky – some of their colleagues were not. By midday, many ICL employees lay trapped and dying, buried beneath tonnes of rubble after the huge explosion ripped through the four-storey building.

Those who had been thrown clear lay on the ground, blood pouring from head wounds, many suffering broken limbs.

Survivors started to crawl from the twisted remains of the building, which had been reduced to a scene which resembled the aftermath of an earthquake.

Mr Gilmour, 50, clutching a bandage to his head and covered in dust, feared for his workmates. “All of the office staff were up on the higher floors, and the boss was having a meeting in the conference room. But I never saw any of them outside,” he said. “I think they must all still be in there, lying under the rubble.”

Up to 50 workshop and office staff were caught in the blast. It was understood a group of about ten employees were holding a meeting in one of the upper floors at the time of the explosion.

David Andrews, 50, was also downstairs in the metal-coating plant at the time of the explosion. He said: “I was trying to help pull people from the rubble. All my colleagues were lying on the ground with cuts and bruises to their heads and broken arms. The whole lot just came down on top of them.”

Derek King, who was on the factory floor too, said survivors were “blinded by smoke and dust”. He added: “There was a massive bang and everything came down on top of me.”

An enormous rescue operation quickly swung into place. Up to 200 firefighters arrived on the scene, backed by scores of paramedics and police officers. A major incident was declared and all five of Glasgow’s main hospitals cleared their operating theatres to make way for the injured. Expert teams from across Scotland were called in to offer assistance, while staff and neighbours dashed to offer assistance.

Ann Brown, 31, was shopping in the nearby Iceland and Lidl supermarkets when she heard the blast. She said: “There were people trapped beneath the bricks and stone, and we could hear them screaming.”

James Rafferty, 25, headed from his home after hearing a “huge blast”. He said: “Some of the people were in a real mess – there was blood trailing behind one woman I know, and lots of head injuries. I saw one woman whose legs were crushed. Everyone was dazed, walking around like zombies. We just did what we could for them.”

Relatives of those who worked at the plant rushed to Grovepark Street, fearing for their loved ones.

Andrew Johnston, 29, whose 51-year-old mother, Linda, works at the factory, said: “I heard about it on the radio and tried to call her but I couldn’t get through to anyone so I rushed down here. My first thought was terror, shock … the not knowing if my mum was OK was horrible.

“When I got here, I spoke to a girl I knew worked with my mum, and she pointed my mum out in the crowd of people. I just ran over to her and hugged her, the relief was unreal.

“My mum said she was on the ground level. She thinks it would be office staff working on the upper levels that would have been affected.”

Not everyone was as fortunate as Mr Johnston – and an hour after the blast, the outlook was grim. Brian Sweeney, the Strathclyde firemaster, said up to 16 people remained unaccounted for. He likened the scene to the devastation of an earthquake. “People will be familiar with earthquake sites in places like Turkey and Afghanistan – what we have here is exactly that. It’s like an earthquake site,” he said

Cars which had been parked in the forecourt of the factory, many crushed and covered by masonry, were removed to allow specialist heavy lifting equipment to be brought on to the scene. And the experts arrived from far and wide to boost the rescue effort – Lothian and Borders Fire Brigade sent specialist appliances and sniffer dogs, and specialists came from Trossachs Search and Rescue and an RAF Sea King rescue helicopter was called on to fly in specialist dog teams and their handlers from RAF Waddington, near Lincoln.

The Scottish Ambulance Service had three special operations teams and a mobile control unit on the scene – and at one point, up to 35 fire appliances were there.

Two tents were erected close to the site, with paramedics and doctors on hand. Casualties were stretchered from the scene and a fleet of waiting ambulances ferried them under police escort to local hospitals.

Some of the most seriously injured were flown by air ambulance to the specialist head injury unit at the Southern General Hospital. More than 500 extra staff were drafted in to Glasgow’s hospitals to cope with the emergency.

The operation appeared to work well. Willie Tullet, the clinical director for North Glasgow Hospitals, said: “The staff are coping remarkably well, they’ve really rallied, the camaraderie is great. They are working under very difficult circumstances.

“We are very fortunate a fire did not ensue, or the outcome could have been much more catastrophic.”

Mr Tullet said the Glasgow-wide incident-response plan had been put into action immediately after the blast and had worked extremely well: “It has worked magnificently well, there have been no glitches whatsoever. The main thing we were worried about was a large number of serious casualties ending up in one hospital but it has balanced out really well.”

Both the Red Cross and the Scotland-based International Rescue Corps – who have experience of earthquake rescue across the world – arrived at the scene to help search for those left under the rubble, using heat-seeking cameras, listening devices and detectors which can pick up breathing.

Graham Payne, the director of the disaster rescue service Rapid UK, explained that fire crews were using a combination of hi-tech equipment and a softly-softly approach in their rescue attempt.

He said: “One of the best things they can do is a hands-on rescue. They’re not going to be subject to earthquake aftershocks like in situations we usually deal with so they shouldn’t get too much more movement from the rubble. So it’s a case of digging in and getting people out. There is always a risk of structural collapse but they’re going to have structural engineers on hand who will advise them so they can secure one part before they move on to the next. So it may be a fairly protracted rescue.”

The lengthy operation – described by the firemaster as “arduous, dangerous and difficult” – took its toll on the emergency services too. Every now and again, tired and drawn firefighters emerged, covered in dust, from the building. A number had suffered heat exhaustion – and one, reportedly, asbestos inhalation – and were being treated in hospital last night.

Those still hunting for survivors called for silence as they tried to hone in on the calls for help from within the rubble. Gradually, the mood among rescuers improved, as one by one, office workers were pulled from the rubble – and fears of a double-digit death toll receded. By 8pm, a seventh survivor, a young woman, was rescued.

By late last night, police had confirmed that four people had been killed as a result of the blast – three died at the scene and one in the city’s Western Infirmary. Another 43 were injured – 17 of them seriously, the remainder were “walking wounded”. It was unclear how many people were left in the rubble, although there were suggestions five people may still be trapped.

With light fading, massive floodlights were brought in to allow the rescue operation to continue overnight to bring out the survivors as soon as possible. A Strathclyde Fire Brigade spokeswoman said: “The number of people trapped is presently unconfirmed. However, fire service personnel are in voice contact with several of those who are trapped in the debris and are attempting to rescue them.”

The firemaster revealed that the emergency services were concentrating their efforts on two pockets beneath the rubble where survivors were still trapped. Mr Sweeney said: “We’re focusing on those people who are currently alive. And let’s get them out alive.

“They’re in various states of ability to talk, some are injured, some are badly injured, some are in shock, some will be traumatised and so we’re having as much decent dialogue as we can with them, trying to comfort and reassure them and offer support under what must be very traumatic circumstances.”

Some of the conversations had been by mobile phone but by about 10pm, verbal contact had been lost. It was thought at least three people were still trapped under the rubble. Rescue dogs were being used to scour the site, looking to pick up the scent of those trapped.

The firemaster said: “We are now entering the second stage of the operation – we no longer have any contact with those we were in voice contact with earlier today. We now enter the stage with listening apparatus and sniffer dogs.

“As long as we feel there is a possibility that someone is still alive, this continues to be a rescue operation. Some people are critical, some are seriously injured. The injuries go right across the scale.

“What we want to do now, with the dogs in, is to focus on those areas where the dogs are becoming agitated or excited, because that could signify a sign of life. At this moment in time, I am not predicting that I will call a halt to the rescue operation within the next three days.”

Patricia Ferguson, the MSP for Glasgow Maryhill, said the area was in a complete state of shock.

“It is devastating for the community. We have now had confirmation of four fatalities. Our best wishes and sympathy goes out to those people who have been bereaved by this accident,” Ms Ferguson said.

As Alistair Darling, the Secretary of State for Scotland, pledged a full inquiry into the tragedy, one employee of ICL Plastics was thankful he had been away from work at the time of the blast.

John Alexander, 27, a spraypainter with the firm, should have been working in the plant, but was instead, coming back from the dental hospital, where he had taken his young daughter for treatment.

He said: “It was the gas that went on one of the ovens. My manager was on the mobile afterwards, saying he was trapped. The guy that was doing my job said the wall blew away and he was thrown through the space – that’s where I would have been.

“I’m feeling lucky to be alive. There’s a good few of my friends who have still not come out and I won’t leave until I see them being pulled from the rubble.”