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

The number and extent of casualties was not immediately known but Labour's international development spokesman Jim Murphy, who was nearby, tweeted: "Terrible here in Glasgow. Helicopter crashed into roof of pub"

The bar is the Clutha bar, widely known as the Clutha Vaults, on the banks of the Clyde.

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.

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."

Mr Murphy 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 on their way to 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".

Nine piece Glasgow ska band Esparanza were playing at the time. 

Eyewitness Gordon Smart said: "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."

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

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.

"It was just a bang, my daughter said it was like a firework explosion, something like that. There doesn't appear to be a fire."

Ms Morris added she thought a police helicopter was involved in the crash.

Mr Murphy said 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".

He said: "I think the building's only about 18ft, 20ft, tall so part of the helicopter is sticking out, but most of the helicopter is in the building."

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