Scores of firefighters on Thursday fought a blaze at Paris' Cite des Sciences, one of the biggest science museums in Europe, the fire brigade said.
The fire, which broke out before dawn in an empty building that was being decorated, took five hours to bring under control, spokesman Gabriel Plus told AFP.
Around 30 fire trucks and 120 firefighters were sent to the scene. Two fighters were treated after being overcome by intense heat.
"It's still red-hot inside," Plus said, cautioning that there was still a risk that the blaze could restart.
"Wooden pallets, cardboard boxes, plasterbloard, electrical cables are burning," he said.
The heat was so fierce that firefighting teams were only able to work for 20 minutes at a time before having to be rotated.
The Cite des Sciences is more a science learning centre for the general public than a traditional museum.
Located in northeastern Paris in the Parc de Villette, on the site of the city's former slaughterhouses, the complex draws around five million people a year.
The museum comprises four huge cube-shaped buildings.
The fire occurred in a 10,000-square metre (107,000-square-feet) cube that was being fitted out for shops, and was due to open on October 15.