GLASGOW (AFP) - Pubgoers inside The Clutha bar in Glasgow told on Saturday how the roof collapsed shortly after they heard a giant thud, as a police helicopter plummeted onto the crowded bar.

In the initial confusion, "I thought 'has the band blown something electrical like a speaker?'" said client William Byrne.

He and others recounted how customers and passers-by helped pull the wounded from the wreckage and tried to shift debris to allow others to escape.

At least one person was killed and 32 others taken by ambulance to city hospitals, police said, as the search for more possible victims continued.

Revellers in the one-storey pub, by the River Clyde in the city centre, were watching Glaswegian ska band Esperanza play when the chopper hit the one-storey bar at around 10:25pm local time on a busy Friday night.

Byrne, 45, from Coatbridge, east of the city, was in the bar with his brother and a friend, when the helicopter hit.

"I was just coming back from the other side of the pub and there was a huge bang," he told BBC television.

"There was a couple of seconds of almost stillness after this bang and then the whole other side of the pub collapsed in on itself, the roof and the gantry of the bar collapsed.

"The side we were in was covered in dust but, from debris, relatively unscathed," he said, with the bar going dark.

"The left hand side of the pub just collapsed. People on our side of the pub were so, so lucky.

"I went outside, we held the door open, trying to make people to file out one by one.

"We've gone back in and helped lift the gantry up and get a few people out." Brendan Riordan, said it had been tough to move in the bar with the amount of people enjoying the concert.

He told the BBC he heard "a very loud bang" before a cloud of dust filled the pub.

"I was on the right side of the pub where the band were performing", adding that the central and left sides had caved in.

"After I exited the pub I saw people coming out covered in blood and covered in dust.

"There were people quite desperate and just before I left the inside of the pub I noticed that the ceiling had fallen towards the bar.

"People were not aware that a helicopter had crash-landed on the pub."

Retired firefighter Edward Waltham ran inside to help with the rescue effort.

"I helped grab a couple of people. One gentleman in particular who was completely covered in dust, who had very shallow breathing and appeared to be quite badly injured." he told the BBC.

"My initial reaction for him from my experience was to try not to move him because he had been in a crush situation.

"But as we were lying there other people were literally being pulled out of the pub and more or less thrown on top of us."

Esperanza, the nine-piece group playing a free gig in the venue, said the group was safe and well.

"Our only concern now is for the safety and welfare of those less fortunate than ourselves," they said on their Facebook page.

"Waking up and realising that it is all definitely horribly real," said Jessica Combe, the band's bassist and general manager.

"Despite the situation everyone was so helpful and caring of each other. The police, ambulances, firefighters all did a stellar job and continue to do so today in extremely difficult conditions."