Officials said more fatalities were expected and crews were in a "search and recovery" operation in the aftermath of Friday's crash, the BBC reported Saturday.
Scotland's First Minister Alex Salmond said during a news conference Saturday it was a "black day for Scotland."
Emergency responders were "working hard to recover people still inside the building," Police Scotland's Deputy Chief Constable Rose Fitzpatrick said Saturday, which is St. Andrew's Day, a national holiday in Scotland.
Chief Constable Stephen House said it wasn't clear whether more people remained trapped inside.
"We are still in a search and recovery phase," House said. "This is a very difficult and sensitive operation."
House said the operation was complicated by damage to the structure and it would take some time to complete a search of the facility and determine how to begin the investigation.
"Clearly the safety of those conducting the search is of the highest importance," House said.
The BBC reported about 120 people were in the pub when the helicopter, carrying two police officers and a civilian pilot, crashed onto the roof. Many either escaped or were rescued, but others were trapped when a part of the building collapsed.
The Clutha, near the banks of the Clyde River, is one of Glasgow's oldest pubs, the BBC said, and is a popular live music venue.
The helicopter crashed as the band Esperanza played, CNN reported.
"Everyone was really confused and scared," Grace Maclean, who was listening to a band when the crash occurred. "No one had a clue. There was no explosion. No bang. It was really quiet."
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