The fire that began on Tuesday in the copper roof of the 17th-century Old Stock Exchange, or Boersen, has spread to much of the building, and parts of the roof have also collapsed, said firefighters' spokesman Jakob Vedsted Andersen.

"The extinguishing work is very difficult," said Vedsted Andersen, adding there are parts of the building the firefighters could not enter because it was too dangerous.

The cause of the fire was not immediately known.

The building, which is next to the Christiansborg Palace where the parliament sits, is a popular tourist attraction.

Its distinctive spire, in the shape of the tails of four dragons twined together, reached a height of 56 metres.

Huge billows of smoke rose over downtown Copenhagen and people were seen rushing inside the building to save paintings.

Danish Culture Minister Jakob Engel-Schmidt said it was touching to see how passers-by helped emergency services "to save art treasures and iconic images from the burning building".

Ambulances were at the scene but there were no reports of casualties.

Members of an army unit were seen being deployed to cordon off the area, and the Danish Emergency Management Agency was also helping out.

The building had been encased in scaffolding.

The roof, masonry, sandstone and spire of Boersen - built in 1615 and considered a leading example of Dutch Renaissance style in Denmark - was being renovated, said the Danish Chamber of Commerce, which moved into the building after Copenhagen's stock exchange left in 1974.

The chamber's head, Brian Mikkelsen, was among those helping to carry paintings out of the building.

"It is a national disaster," Mikkelsen told reporters.

The adjacent Christiansborg Palace has burned down on several occasions, and most recently in 1990 a fire broke out in an annex of the Danish parliament, known as Proviantgaarden.

However, the Old Stock Exchange survived unscathed.

That annex, which lies in the block behind the Old Stock Exchange, was evacuated as a precaution, as were different ministries in the street behind the burning building.

Beside housing the Chamber of Commerce, the Old Stock Exchange is used for gala dinners, conferences, parties and other events.

Australian Associated Press