Firefighters rescued a window cleaner dangling from the side of a tall building in St. John's on Thursday after his safety line failed.

The man, who was cleaning windows outside The Rooms Provincial Art Gallery, dangled almost 30 metres from the ground for an hour and a half before being rescued.

An eyewitness said the window cleaner dropped down, stopped suddenly, and then banged on the glass and asked the people inside to call for help.

Don Byrne, with the St. John's regional fire department, said the man was trapped.

"It appears that one of his ropes may have broken and he dangled from there," said Byrne.

"Once he dangled, that was it, he just sat there. He wasn't going anywhere, up or down."

Dozens of people looked on as firefighters tried to reach the man with an aerial ladder. However, the ladder was not long enough, so a high-angle rescue team was used instead.

Reaches dangling worker

Firefighter Charlie Quick was sent over the ledge and lowered down the side of the building.

Once he reached the cleaner, Quick said, he put a safety harness on the man.

"He was really calm, which was great," he said.

"He's at that for a living, hanging off the side of buildings ... he was really good, I must say."

Onlookers cheered and applauded from the sidewalk below when the pair was lowered to safety.

The rescued man, who declined to speak with the media about his ordeal, refused a stretcher and walked away.