The blaze ripped through Copenhagen's 400-year-old landmark stock exchange on Tuesday, toppling its spire and triggering the collapse of a large roof section, in scenes reminiscent of the 2019 fire in Paris.

Firefighters would stay on site for at least another 24 hours, Copenhagen's fire department said on Thursday.

The flames that flared overnight were primarily in the basement of the building and were quickly put out, officials said.

As attention turned to how the building could be restored, the mayor of Copenhagen, Sophie Haestorp Andersen, said she would travel to Paris in May.

"Yesterday, I reached out to the mayor of Paris, Anne Hidalgo, and talked about their experiences from when Notre-Dame burned five years ago," Andersen told Reuters.

"She invited me and a team to Paris to look at how they dealt with the situation there."

In parts of the building only outer brick walls remained as everything else had burned, while the floors of some of the sections still standing were waterlogged from efforts to douse the fire, reporters allowed inside on Wednesday said.

"The work to stabilise the freestanding outer walls will continue for most of the day," the Greater Copenhagen Fire Department said on X.

Large containers filled with bricks were being stacked to support the damaged walls and prevent the historic brickwork from collapsing.

The Danish Chamber of Commerce, which owns the building and uses it for its headquarters, has vowed to rebuild the iconic exchange, which originally opened as a commodities trading venue in the 17th century.

The building no longer houses the stock exchange.

Police investigating the incident said it could take months to determine the cause. No one was hurt in the blaze.

Australian Associated Press