Danish firefighters were still at work extinguishing the last pockets of a fire that destroyed a 400-year-old Copenhagen landmark a day after the blaze began.

“This is sad, so sad,” said Mikkel Jensen, a 44-year-old civil servant, as he looked up at the twisted scaffolding still clinging to the ruins of the city's Old Stock Exchange.

Morten Langager, manager of the Danish Chamber of Commerce, which was headquartered in the Old Stock Exchange and owned the building, said the building, built in 1615 and known for its green copper roof and distinctive 56-meter (184-foot) spire in the shape of four intertwined dragon tails, should “rise again.”

His boss, Chamber head Brian Mikkelsen, said Tuesday that “no matter what, we will rebuild Boersen” and that the board had backed the idea.

No decision has yet been made about who will reconstruct the building, which would cost millions, if not billions of kroner (dollars). A cautious estimate said it could take up to 10 years.

Many in Denmark compared Tuesday's fire to the April 2019 blaze at Notre Dame that destroyed the 800-year-old cathedral's spire. Its restoration is slated for completion this year.

The extent of the damage, caused by flames and the tons of water poured to extinguish them, was still unknown.

“A lot has disappeared in the fire,” Tim Ole Simonsen of the Greater Copenhagen Fire Department said of the building. “The stability of external walls is threatened by the fact that the wooden structure that was holding them is gone.”

Up to 40 shipping containers, filled with concrete, will be put around the remaining outer walls to stabilise it, the fire department said.

Police hadn't yet been able to enter the building to begin investigating the cause of the fire, said Brian Belling, a Copenhagen police officer in charge of the investigation. The blaze is believed to have started on the building's roof during renovations on Tuesday morning.

“It can take a long time to find the cause of a fire,” Belling said.

When the fire engulfed the building on Tuesday, passers-by, Chamber of Commerce staff, police officers and members of an army unit that had been sent to help raced inside the building to save its treasures.

Mikkelsen, who was one of the employees who ran in, told public broadcaster DR that the building had made a plan for what to save in a “worst case” scenario. When it was too dangerous for people to go up to the first floor, firefighters with breathing gear were sent in to retrieve priceless items, he said, adding that many of the building's most valuable contents, which included priceless paintings and other works of art, had been saved.

The Chamber of Commerce said that among the saved items was a huge painting titled, “From Copenhagen Stock Exchange,” completed in 1895 by Danish artist P.S. Krøyer.

An hour after the first reports of fire, it rapidly engulfed the spire and sent it crashing onto the building, which was shrouded by scaffolding, and onto the street behind.

Huge billows of smoke rose over downtown Copenhagen and could be seen from southern Sweden, which is separated from the Danish capital by a narrow waterway. Ambulances were at the scene but there were no reports of casualties.

Smoke damage closed ministries located in the street behind the Old Stock Exchange, which remained shut Wednesday as employees were told to work from home because of a strong smell of smoke in the buildings.

The buildings must be thoroughly cleaned and their ventilation systems must be checked and perhaps replaced before ministry staff can return, said Rasmus Brandt Lassen, head of the Danish Building and Property Agency.

“We have told them that they should expect to work at home for the rest of the week,” Brandt Lassen said,

The exchange is considered a leading example of the Dutch Renaissance style in Denmark. The Chamber of Commerce moved into the building after Copenhagen's stock exchange left in 1974.

Its dragon spire was one of the many of topping the city's churches and castles, which have earned Copenhagen its nickname as “the city of spires.” Others copper-covered belfries include the serpentine spire of Our Saviour's Church, those of the downtown Renaissance Rosenborg Castle, and the tower of the Christiansborg Palace which houses the Danish parliament.

A main road running past the Old Stock Exchange remained closed Wednesday.