There are a number of casualties, police said.
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Scottish First Minister Alex Salmond said people should prepare for the likelihood of fatalities while emergency workers labored to free people trapped under rubble in the shadow of the mangled helicopter, which was embedded in the pub's roof.
Scene of helicopter crash (Photo: AP)
(Photo: Getty Images)
Visibly shaken eye witnesses described scenes of horror as the 12-meter (40 ft) helicopter with two police officers and a civilian pilot on board spiraled into the Clutha pub in a busy area of Scotland's biggest city, collapsing part of the roof.
"It was fairly busy, we were all having a nice time and then there was like a whoosh noise," Grace MacLean, who was in the pub at the time of impact, told the BBC.
"There was no bang, no explosion and then there was what seemed like smoke and we were all joking that the band had made the roof come down and then it started to come down more and someone started screaming and the whole pub filled with dust and you couldn't see anything, you couldn't breath."
Police said it was too early to speculate on what caused the Eurocopter EC135 T2 to smash into the pub at 10:25 pm local time where more than 100 people were listening to a band perform. The helicopter did not appear to have caught fire.
"Given an incident of this scale we must all prepare ourselves for the likelihood of fatalities," said Scotland's leader, Salmond.
Scottish Police Service Deputy Chief Constable Rose Fitzpatrick said she could only say that there were a number of casualties and that emergency workers were still trying to free people from the pub some five hours after the crash.
One emergency worker said he had been in contact with people trapped at the scene.
Caked in dust and blood, revelers rushed into the street immediately after the crash in what a Member of Parliament from the opposition Labor party described as a "horrible scene" on the bank of the River Clyde.
Arriving by chance at the scene just moments after the crash, MP Jim Murphy praised bystanders who formed human chains to rescue people from the 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," said Murphy, who had blood on his shirt.
British Prime Minister David Cameron and opposition leader Ed Miliband said their thoughts were with those affected by the crash.
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