Eight people have died and more than 30 people have been taken to hospital after a police helicopter crashed on to the roof of a crowded pub.
Firefighters worked through the night to rescue people trapped inside the Clutha pub in Glasgow after the roof collapsed on more than 100 revellers.
Police Scotland said three of the bodies were found within the helicopter, the BBC reported.
A further 14 people are being treated for "very serious injuries" in hospitals across the city, as tales of heroism continue to emerge from the shocking scenes.
It's not yet known how many are still trapped inside the building.
The shocking scene this morning
— Louise Robertson (@LouMRobertson) November 30, 2013
— Louise Robertson (@LouMRobertson) November 30, 2013
A major incident has been declared and First Minister Alex Salmond said that today "is a black day for Glasgow and Scotland but it is also St Andrew's Day and we can take pride in how we respond to adversity.
"The response from our emergency services and citizens has been exemplary."
I can confirm that it is a police helicopter which has been involved in the tragic accident in Glasgow. #Clutha
— FM Alex Salmond (@AlexSalmond) November 30, 2013
Witnesses described how the helicopter dropped out of the sky and on to the roof of the pub in Stockwell Street "like a stone."
Assistant Chief Officer Lewis Ramsay, of the Scottish Fire and Rescue Service, said firefighters had made contact with people trapped but it was difficult to tell how many were inside as the building was "very unsafe".
"We've had some contact and we're working away just now to make sure that the building is safe in order to get people out," he said.
"We are determined that we are going to get the building stable and we will be in there to carry out those rescues."
The "methodical" search and rescue operation involves 125 firefighters including crews trained in shoring up unstable buildings, excavating into collapsed structures and seeking and identifying casualties using specialist equipment.
John McGarrigle, 38, fears for the safety of his father, also John McGarrigle, 59, who was in the pub last night.
He was waiting at the police cordon near the pub hoping for news.
He said an eyewitness told him that his father, also John McGarrigle, 59, was sitting "right in the spot" where the helicopter came down on the building in Glasgow city centre last night.
Speaking to BBC News from the scene, Mr McGarrigle said his father is "probably lying underneath that helicopter".
He said: "The realisation, and just a deep instinct ... kicked in right away as soon as I heard there was an accident at Clutha. I just knew something bad had happened to him.
"When I came round and seen where the position of the helicopter (was) that was when I knew, because he sat in that spot all the time, where the 'copter hit.
"I am still shaking."
— JanHollands (@Janney_h) November 30, 2013
Those nearby the scene united during the immediate carnage, with reports emerging from eyewitnesses that brave Glaswegians ran into the crumbling pub to help.
Labour's international development spokesman Jim Murphy MP said he saw people "clambering out" of the bar as he was driving past and jumped out to help.
He said: "There were people with injuries. Bad gashes to the head. Some were unconscious. I don't know how many.
"The helicopter was inside the pub. It's a mess. I could only get a yard or two inside. I helped carry people out."
Mr Murphy told Sky News people formed a human chain to help pass unconscious people out of the pub so that "inch by inch, we could get the people out."
The more I hear, the prouder I am to be Scottish. Emergency services and revellers working shoulder-to-shoulder saving lives @ScottishSun
— Gordon Smart (@gordonsmart) November 30, 2013
Wesley Shearer, who was inside the pub at the time, described the initial rescue attempts at the Clutha.
"I tried my best to help pull a few people out from the other side of the pub who were trapped but there were other people there who were risking their lives to help those in need," he told BBC Breakfast.
"A chain was sort of formed to pass casualties back after pulling bits of beams and wood out of the rubble - there was beams and wood everywhere and at that point we just started pulling people out, there were other people further inside the pub passing people out to us."
The bar was busy with revellers enjoying a gig by nine-piece ska band Esperanza at the time of the crash.
Brendan Riordan, who was inside the pub at the time of the crash, told BBC News it had been "packed".
He said: "It was quite hard to move in there with the amount of people enjoying the gig."
There couldn't have been many worse nights for the #Clutha to have happened. The no of times we went & to busy to get through door on a Fri
— Del (@OwlAndKitster) November 30, 2013
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Grace MacLean, who was also inside the pub, told BBC News: "They carried on playing and then it started to come down more and someone started screaming and then the whole pub just filled with dust. You couldn't see anything, you couldn't breathe.
"People started coming out with injuries and blood and everyone was going over and trying to help out.
"It was a real testament to the people of Glasgow, everyone in that pub was shouting 'here's the door', they were helping each other out."
— Vincent McAviney (@VMcAVSKY) November 30, 2013
— JanHollands (@Janney_h) November 29, 2013
The band playing at the pub at the time of the crash have now released a statement on their Facebook page, praising the emergency services for their work.
Gordon Smart, editor of the Sun's Scottish edition, saw the helicopter come down from a multi-storey car park around 250 yards away.
He told Sky News: "I was in a car park and looked up and saw a helicopter which I think was a police helicopter.
"It was just such a surreal moment. It looked like it was dropping from a great height at a great speed. I'm about 80% sure that it was a police helicopter.
"There was no fire ball and I did not hear an explosion. It fell like a stone. The engine seemed to be spluttering."
Emergency services at the scene last night
Prime Minister David Cameron today expressed his "deepest sympathies" with the families and friends who lost a loved one in the Glasgow helicopter crash and he thanked the emergency services who "worked tirelessly" throughout the night and also paid tribute to the bravery of "the ordinary Glaswegians who rushed to help".
The Queen has also said her thoughts and prayers are with the victims of the police helicopter crash in Glasgow.
Scotland waking up to the saddest of St Andrew's days. Thoughts with all those still at the scene of this awful tragedy
— Nicola Sturgeon (@NicolaSturgeon) November 30, 2013
Many families waking up this morning fearing worst & police family amongst them Many thanks 4 messages of support from across world #Clutha
— ScotsPolFed (@Scotspolfed) November 30, 2013
The Scottish Ambulance Service confirmed that multiple ambulance crews and the Special Operations Team were at the scene.
One image of the crash showed the dark blue helicopter on the roof with yellow ''POLICE'' insignia on part of the wreckage.
Police Scotland have confirmed the aircraft involved was a Eurocopter EC135 T2 with two police officers and a civilian pilot on board.
Something "dramatic" suddenly occurred to cause the police helicopter crash in Glasgow, according to an aviation safety expert .
— Emily McCulloch (@emmyloumac) November 30, 2013
Dreadful news about the #Clutha, a great, friendly wee pub, and one of my dad's favourites. Thoughts and prayers with everyone involved.
— Stephen Sullivan (@SullivanSK) November 30, 2013
A large section of the city centre was cordoned off with all roads leading to the junction of Clyde Street, Stockwell Street, Bridgegate and the Victoria Bridge closed.
Helicopter operator Bond Air Services said it was working with Police Scotland. A full probe involving the Air Accidents Investigation Branch (AAIB) and the emergency services is under way, but an inquiry could take "months."
Police have set up a telephone number for members of the public who are concerned about relatives who may have been involved in the crash. It is 0800 092 0410.