Firefighters address scaffolding hanging outside of fire-damaged Danish landmark.

COPENHAGEN, Denmark—Firefighters stabilizing the burned-out husk of Copenhagen’s historic Old Stock Exchange tried to use giant shears on a crane Friday to cut away dangling scaffolding, but suffered a setback when the implement fell into the debris.

A large blaze broke out Tuesday at the 400-year-old landmark in Copenhagen, toppling its roof and iconic dragon-tail spire. On Thursday, a large section of the outer wall of the building collapsed inwards. The most valuable paintings and items inside had been saved from the flames, and no one was injured.

“A further building collapse is absolutely a possibility,” said Tim Ole Simonsen, a spokesman for the Greater Copenhagen Fire Department, adding that structural engineers were assessing how best to remove precarious remnants of scaffolding. “It is very difficult work.”

He later said the work was paused and “our efforts have taken a new turn,” after the powerful shears mounted on the end of a crane had fallen into the ruins of the building. There also were still small pockets of fire to extinguish and “tons” of debris to remove, he said.

Brian Mikkelsen, who heads the Danish Chamber of Commerce that owns the 1615 building and was headquartered there, said 45 percent of the structure had been saved, and he underlined pledges made earlier in the week to completely rebuild the structure.

“ It will be rebuilt as it was,” Mr. Mikkelsen said as Danish Prime Minister Mette Frederiksen unexpectedly visited the site Friday afternoon to thank the firefighters, saying their quick response on Tuesday was “impressive” and to give “huge, huge respect.”

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Ms. Frederiksen also supported the idea of rebuilding the Old Stock Exchange but said it was too early to say whether the government would take part in any financing.

“We need to get a better overview, like for instance the insurance money,” Ms. Frederiksen said.

Tuesday’s blaze was believed to have started on the roof of the building, which had been wrapped up in scaffolding while the building was being renovated. The cause remained unclear, and police had yet to enter the burned part of the building to investigate.

The plans to remove the scaffolding was an attempt to salvage the unharmed part of the Copenhagen landmark. Firefighters said they could not say how long that would take, and adjacent streets were still closed for traffic.

No decision has yet been made about who will finance a reconstruction, a project that would cost millions, if not billions of kroner (dollars) and take years.

The exchange, known for its green copper roof and distinctive in the shape of four intertwined dragon tails, sits on the waterfront next to the Danish parliament. It is considered a leading example of the Dutch Renaissance architectural style in Denmark. The Chamber of Commerce moved into the building after Copenhagen’s stock exchange left in 1974.

By Jan M. Olsen