Firefighters battled a blaze at Copenhagen's historic former stock exchange for a third day on Thursday and said their work would continue for at least another 24 hours.

Half of the 17th-century Borsen building was destroyed and its 54-metre (180-foot) spire tumbled to the ground in the fire that broke out early Tuesday, in scenes that shocked Denmark.

"Work has continued ... We have extinguished a few small fires in the building, primarily in the basement," rescue services wrote on X.

"The work to stabilise the free-standing outer walls will continue for most of the day. We expect to be present at the scene of the fire for at least another day."

Police said several streets around the building would remain inaccessible until Monday.

The fire began under the copper roof of the building, which was undergoing renovations ahead of its 400th anniversary.

The cause of the blaze was unknown and Copenhagen police said Wednesday that a major investigation had been launched.

"It is a complicated process and it can take several months before we can reach an answer," police said in a statement.

As one of the capital's oldest and best-known landmarks, the Borsen building housed the Danish Chamber of Commerce as well as a vast art collection.

Several hundred artworks were rescued from the fire.

Brian Mikkelsen, the director of the Danish Chamber of Commerce said Wednesday the top of the building's iconic spire -- designed to look like it was made up of the intertwined tails of four dragons -- had been recovered intact.

"It gave a glimmer of hope. Because it will once again adorn our beautiful workplace and Copenhagen," Mikkelsen said in a post to X.

Located close to the Christiansborg parliament and seat of government, the building was commissioned by King Christian IV and built between 1619 and 1640. It was the stock exchange until the 1970s.