Four people have died after a huge blast devastated a plastics factory in the west end of Glasgow, police said.
The blast destroyed the building
Seven survivors have been pulled from the rubble of the Stockline Plastics building in Grovepark Street.
Forty people have been injured, 16 seriously, and some firefighters have been treated for heat exhaustion.
Strathclyde Firemaster Brian Sweeney said the rescue operation was entering its second phase as sniffer dogs were brought in to search for survivors.
He said firefighters were no longer in contact with anyone in the rubble - but stressed there was cautious optimism that more people would be found alive.
"We still think there may be people in there alive and as long as we still think that we will still keep working," he said.
Experience from earthquakes and building collapses in places like Turkey and Afghanistan had shown that people could be rescued alive after days or weeks.
Stobhill Hospital - 14 people
Glasgow Royal Infirmary - nine people
Western Infirmary - 11 people
Southern General - five people
Victoria Infirmary - one person
"As long as we feel there is a possibility that someone can be rescued this remains a rescue operation not a recovery operation," he said.
Mr Sweeney said it would be "unprofessional" to speculate on the number of people who were still in the debris.
He said it had been "difficult, tough, punishing, emotional work" for fire crews.
About 100 people worked in the four-storey building, which was all but destroyed in the blast.
The explosion took place shortly after 1200 BST and was heard by people several miles away.
The cause of the blast is under investigation, although eyewitnesses reported that an industrial gas oven had exploded.
An information line providing regular updates for concerned relatives and others affected by the blast has been set up on 0870 9009586.
Mr Sweeney estimated that the rescue operation would last for at least 48 hours.
He said earlier that some of those trapped had been speaking to emergency services on mobile phones.
The people pulled from the debris included a young woman who had been on the third floor.
He said she suffered serious crush injuries in the plunge to the ground floor, but it was hoped she would make a full recovery.
Mr Sweeney said rescue teams were using thermal imaging cameras and heat-seeking equipment.
Extensive work was required to stabilise the building, which was in a precarious position.
"This is a slow and a painstaking process," Mr Sweeney.
"Many of the members of the public will be familiar with earthquake procedures in Turkey and Afghanistan
so what we have here is a scene very similar to that."
Glasgow's Lord Provost, Liz Cameron, said the explosion had come as a "terrible shock".
"Events such as this - rare though they are, thankfully - tend to bring the people of Glasgow together and I am sure they will join with me in expressing our deepest condolences and sympathy to all concerned.
"Glasgow City Council staff are on site to assist the emergency services, and will continue to do so until further notice," she said.
Local Councillor Hanzala Malik said the factory had been cleared as safe only a couple of months ago.
"For an explosion to happen and people's lives to be put at risk is just incredible and unbelievable.
"Everybody is just stunned and hoping for the best for the people who are trapped."
Police said that 41 people had been injured in the incident, 17 of them seriously.
The Scottish Ambulance Service said it had three special operations teams and a mobile control unit which specialises in handling major incidents at the scene of the blast, along with 16 ambulance crews.
An RAF Sea King rescue helicopter from North Yorkshire was used to fly specialist dog teams and their handlers from RAF Waddington near Lincoln to Glasgow.
The Health and Safety Executive said it would be carrying out an investigation into the explosion.
A number of inspectors are now at the scene, but their inquiry cannot get under way until the site has been made safe by the fire service.