It was unclear how many people were still inside, Asst. Chief Officer Lewis Ramsay with the Scottish Fire and Rescue Service said. He said teams including 125 firefighters on the scene were working to stabilize the building and "get people out."
A crew of three two police officers and a civilian pilot were aboard the Eurocopter EC135 T2, according to Scottish police. Deputy Chief Constable Rose Fitzpatrick said it was too early to offer details on why the helicopter came down.
It "fell like a stone," Gordon Smart, editor of the Scottish edition of the Sun newspaper, told Sky News. "There was no fireball and I did not hear an explosion. The engine seemed to be spluttering."
Grace MacLean, who was inside the pub at the time, said she heard a "whoosh" noise and then saw smoke.
"The band were laughing, and we were all joking that the band had made the roof come down," she told the BBC. "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 formed a human chain to help pass unconscious people out of the pub so that "inch by inch, we could get the people out," said Labour Party spokesman Jim Murphy, who was in the area when the helicopter came down.
"The helicopter was inside the pub. It's a mess. I could only get a yard or two inside. I helped carry people out," Murphy told Sky News. "I saw a pile of people clambering out of the pub in the dust. No smoke, no fire, just a huge amount of dust."
He called it "a horrible, horrible scene."
Prime Minister David Cameron tweeted: "My thoughts are with everyone affected by the helicopter crash in Glasgow - and the emergency services working tonight."
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Follow Cassandra Vinograd at http://twitter.com/CassVinograd
