Witnesses said they saw the helicopter dropping from the sky. The people inside Clutha Club heard a whooshing sound, after which the roof of the pub caved in. Following the incident, the pub was filled with dust and smoke, witnesses said.
Sources said it was a police helicopter which crashed into the single storey pub on the banks of river Clyde.
Many casualties have been reported, said sources. Rescuers formed a human chain to help victims get out of the debris and get medical help.
Dozens of fire fighters and medical officials have reached the spot, sources said.
British Prime Minister David Cameron told AP news: "My thoughts are with everyone affected by the helicopter crash in Glasgow - and the emergency services working tonight."
Grace MacLea was one of the people who were inside the pub at the time of the accident. He narrated the incident to BBC news, "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.
"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."
(With agency inputs)