"The crash site has been fully cordoned off"
At least three people have died after a police helicopter crashed into a busy pub in Glasgow, the BBC has been told.
A source close to the investigation said the final number of fatalities would almost certainly be higher.
The crash happened at The Clutha in Stockwell Street at 22:25 on Friday. People are still thought to be trapped.
There were three people on board the helicopter - two officers and a civilian pilot. Thirty-two people have been taken to local hospitals.
A senior fire officer said they had made contact with some people trapped inside the pub but the building was unsafe and they were taking a "methodical" approach to the rescue.
The emergency services could be seen on the pub's roof trying to rescue people from inside.
It has been reported that about 120 people were in the pub at the time of the crash. Many were rescued or escaped but others have been trapped by a collapse on the left-hand side of the building.
Emergency services have erected barriers around the scene and specialist rescue teams are in the pub with sniffer dogs.
Some of the injured were taken to a nearby Holiday Inn Express, while more serious casualties were being treated in hospital.
As he left the accident and emergency department of Glasgow Royal Infirmary, a staff member who did not want to be interviewed was asked how serious the injuries were. He replied: "Very".
Glasgow's Health Board said it had put in place its "well-rehearsed major emergency arrangements" and that local hospitals had been on "immediate standby".
Emergency services said there have been
A large area of the city centre has been cordoned off.
Images of the crash showed the wreckage of a dark blue helicopter with a yellow "Police" insignia lying on the pub's roof.
Police Scotland Deputy Chief Constable Rose Fitzpatrick said: "At 22:25 on Friday evening, the Police Scotland helicopter - a Eurocopter EC135 T2 - came down on the roof of the Clutha Vaults pub in Stockwell Street, Glasgow.
"There were three people on board - two police officers and a civilian pilot. There were a number of customers inside the bar at the time."
She said the rescue operation was ongoing and it was too early to provide any details around why the helicopter came down.
Helicopter operator Bond Air Services said it was working with the police and emergency services.
Jim Murphy, the Labour MP for East Renfrewshire, was in the area at the time of the crash and said he ran into the pub to help before emergency services arrived.
He told the BBC there was "pandemonium" as people tried to get out of the pub.
"It was almost like slow motion," he said, adding: "People just formed a bit of human chain, side by side with each other, to help pull injured people out."
The shadow cabinet minister, who had blood on his shirt which he said was not his, described what he saw as a "horrific scene".
I can't really believe what's happened. I was on the
phone at the time and I heard a misfiring engine.
Looked above me, couldn't work out where it was
coming from, it got louder and louder and I just spotted
a helicopter falling from the sky.
It was falling at great speed. It looked like the rotors
weren't spinning but the helicopter was sort of turning
in a strange position and dropping at great speed.
Oddly enough there was no explosion no fire ball.
It's a busy Friday night, a lot of people have been out in
the pubs, there are a lot of pubs in that area. I do fear.
I would be staggered if there weren't fatalities from the
area it's landed.
Eyewitness Fraser Gibson, 34, was inside the pub with his brother to see his former band, Esperanza.
"Midway through their set it sounded like a giant explosion," he told BBC Scotland.
"Part of the room was covered in dust. We didn't know what had happened. We froze for a second; there was panic and then people trying to get out the door."
Mr Gibson added: "I would say there was maybe 120 people inside the pub. A lot of people managed to get out straight away, but it was hard to tell how many were actually trapped in the other half of the bar.
He said there had been no indication a helicopter had caused the devastation, adding: "The roof had just totally collapsed.
"There were shards of wood sticking out the top but nothing that said there had been a helicopter crash."
Eddie Waltham, a former firefighter who had a friend inside the pub, told the BBC: "A roof joist came down and hit him and pushed him towards the window which is at the left side of the left door."
He added later: "My own reaction was to run straight up to the pub.
"It was amazing to watch just how people were trying so hard to get into this building."
John McGarrigle who said he feared his father had been in the pub at the time said: "I've checked every hospital and there's no sign of him. I'm very anxious.
"I'm just going to stand here till I see casualties come out of the building."
First Minister Alex Salmond tweeted: "The emergency services are in full operation. Our thoughts are with everyone involved. Scottish resilience operation now mobilised."
Prime Minister David Cameron tweeted: "My thoughts are with everyone affected by the helicopter crash in Glasgow - and the emergency services working tonight."
In 2002, a police Eurocopter EC-135 came down in a field in Ayrshire. All three people on board survived.
In 1990, a police sergeant was killed when a Bell Jet 206 helicopter crashed in bad weather at Newton Mearns in East Renfrewshire.