The Eurocopter EC135 T2 chopper had a crew of three consisting of two police officers and a civilian pilot as it came down on the Clutha Vaults pub last night on the bank of the River Clyde, which was packed with more than 100 people at the time who were listening to a performing band.
"There were three people on board the helicopter, two police officers and a civilian pilot, and on a busy Friday night, there were a number of customers in the bar," said Rose Fitzpatrick, deputy chief constable, Police Scotland.
"We are working hard to recover people still inside the building and we will make further details available when we have them," she said.
"A full investigation is now under way however at this early stage it is too early to provide details on why the helicopter came down," she added.
Investigators from the Air Accident Investigation Branch (AAIB) arrived on the scene this morning trying to piece together what had happened.
Alex Salmond, the First Minister for Scotland, had posted a message on Twitter saying we should "prepare ourselves for the likelihood of fatalities".
Nicola Sturgeon, Deputy First Minister of Scotland, added, "Absolutely awful news about a helicopter crashing into the Clutha. All my thoughts are with everyone involved and the emergency services."
British Prime Minister David Cameron tweeted, "My thoughts are with everyone affected by the helicopter crash in Glasgow - and the emergency services working tonight."
Police have confirmed that 32 people, with "multiple types of injury" had so far been taken to hospitals across the city - Victoria Infirmary, Glasgow Royal Infirmary and the Western Infirmary - following the crash.
"Emergency services responded immediately and remain at the scene at the present time. Cordons have been put in place to allow the rescue operation to take place," a Police Scotland spokesperson said.
Jim Murphy, former shadow defence secretary and a Labour frontbench MP who was driving by and stopped to help, said members of the public formed a human chain to get people out of the building.
"What I saw was a pile of people clambering out of the pub, dust everywhere. People were covered in multiple injuries," he said.
Eyewitnesses inside the bar at the time of the crash have said that in the moments after the helicopter came through the roof there was no explosion.
Grace MacLean, who was inside the pub at the time, told the BBC, "It was fairly busy. We were having a nice time and then there was just a whoosh noise. There was no bang, no explosion. We were all joking that the band had caused the roof to come down. They carried on playing at first."
Gordon Smart, editor of the Rupert Murdoch-owned Sun newspaper's Scottish edition, saw the crash from a multi-storey car park nearby.
"The thing that was disturbing and shocking was there was no explosion. I couldn't understand why a helicopter would fall from that height and not explode. To see the angle, the speed and the trajectory of the fall, it was a horrific sight," he said.
Members of Esperanza, the band playing at the time, all escaped from the wreckage unharmed.
They posted on their Facebook site, "Best wishes to everyone from tonight. Hope everyone who got out managed to get home or somewhere safe to stay. To everyone who was injured. Please get fixed soon. And please please please let us know of the people who we haven't heard from yet. Hope they're found ok."
Police helicopters in Glasgow use a small heliport further down the Clyde in Finnieston, close to the Scottish Exhibition and Conference Centre, around a mile west of the scene of the crash.
One witness who lives in the Gorbals area of the city, on the opposite bank, said he had heard a helicopter flying low for some time before the crash.
Today is Scotland's National Day, referred to as St Andrew's Day in honour of the region's patron saint.
A number of events linked to the annual feast day are being cancelled as the tragic toll of last night's crash starts to emerge.