The chopper smashed through the roof of the crowded Clutha Pub, where more than 100 revellers had been listening to a band on Friday night ahead of St Andrews Day, which celebrates Scotland's patron saint.
Witnesses said the helicopter with two police officers and a civilian pilot on board dropped like a stone, while people inside the pub heard a whoosh before the roof caved in and the air filled with dust and screams.
"It's a case of working hard within the building to try and determine how many casualties are there," Fire brigade officer Lewis Ramsay told reporters.
"We are determined that we are going to get the building stable and we will be in there to carry out those rescued."
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Ramsay said the 125 firefighters at the scene had "rescued numerous casualties" who had "multiple types of injuries".
Thirty-two people have been sent to hospitals across Glasgow, police said in a statement today.
"Given an incident of this scale we must all prepare ourselves for the likelihood of fatalities," he said.
An AFP photographer at the scene said the helicopter appeared to have smashed through the top of the single-storey pub on the banks of the River Clyde, with a rotor blade sticking out of the roof.
Police officer Rose Fitzgerald said it was too early to say why the Eurocopter EC135 helicopter crashed.
Witnesses told of confusion, terror and then bravery after the accident.
Grace MacLean, who was inside the pub at the time of the crash, told BBC News that the revellers were listening to a Ska band at the time.
"We were all just having a nice time and then there was like a 'whoosh' noise -- there was no bang, there was no explosion," she said.
"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.