A police helicopter has crashed into the roof of a Glasgow pub, raising fears of multiple injuries.

The number and extent of casualties at the Clutha pub on the banks of the Clyde, widely known as the Clutha Vaults, was not immediately known but First Minister Alex Salmond said in a tweet early today that there was a likelihood of fatalities.

One image of the crash on social media sites showed the dark blue helicopter on the roof with yellow "POLICE" insignia on part of the wreckage. Police Scotland confirmed early today that its aircraft was involved.

The police helicopter is staffed by a civilian pilot and two officers.

The Police Roll of Honour Trust tweeted "Our thoughts are with the crew of @policescotland SP99 helicopter that has crashed in Glasgow - hoping everyone is alright."

Labour MP Jim Murphy, who was nearby when the crash happened, said: "It's a well established Glasgow pub. It's a horrible horrible scene, but well done to the folk who were here. Everyone formed a chain of people from inside the pub to outside, and the fire brigade and everyone were here very quickly."

The Scottish Fire and Rescue Service said they were at a "building explosion" in Glasgow's Clyde Street.

A spokeswoman for Strathclyde Fire and Rescue Service said: "I can confirm that we have people at the scene.

"It is obviously a major incident. There are numerous fire engines there; 15 fire engines at the moment along with specialist services.

"I cannot confirm if there have been any injuries."

Fraser Gibson, who was in the pub, said: "I just felt an explosion from the other side of the bar and the roof just caved in."

Speaking about the reports of a helicopter crash, he said: "The force of it suggested the possibility that there was another factor involved and somebody did mention a helicopter as soon as we came out but I couldn't see anything."

Another eyewitness described hearing a "loud crack".

Grace MacLean, who was inside the pub at the time of the crash, said: "There was a ska band on in the pub just at the back and it was fairly busy.

"We were all just having a nice time and then there was like a 'whoosh' noise - there was no bang, there was no explosion - and then there was some smoke, what seemed like smoke. The band were laughing and we were all joking that the band had made the roof come down.

"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.

"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."

A message on band Esparanza's Facebook page read: "Thanks for all the messages. I've just spoken to Jake and Jason. It seems that the band are all OK. Not so sure about everyone else."

Ms MacLean said: "People started coming out with injuries and blood and everyone was going over and trying to help out.

"I don't think it crashed, if it had crashed there would have been fire, there would have been a noise, but we didn't hear anything, it was the smoke that we noticed.

"I think maybe whoever was in the helicopter just tried to land on the roof or something. The propellers could have been on but everyone seemed to be okay.

"I don't know if anyone is trapped in there but there wasn't anyone screaming for someone."

Eyewitness Gordon Smart 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."

Mr Murphy said: "I got out of my car and just tried to help people.

"I saw a pile of people clammering out of the pub in the dust. No smoke, no fire, just a huge amount of dust."

Claire Morris, who lives near the Clutha bar, said: "We heard this bang. We didn't really know what had happened and then we heard people coming out and screaming.

"I wasn't sure whether there had been an explosion. My daughter said to me it was a helicopter that had hit the roof.

"Police are everywhere. We are just very shaken."

She added the pub was very popular and would have been busy on a Friday night.

First Minister Alex Salmond tweeted: "The emergency services are in full operation. Our thoughts are with everyone involved. Scottish resilience operation now mobilised."

Deputy First Minister Nicola Sturgeon tweeted: "Absolutely awful news about a helicopter crashing into the Clutha. All my thoughts are with everyone involved & the emergency services".

Prime Minister David Cameron tweeted: "My thoughts are with everyone affected by the helicopter crash in Glasgow - and the emergency services working tonight."