At least 29 people were injured after a blast rocked the fifth arrondissement of Paris, resulting in a fire affecting some buildings on Wednesday.
Paris police said at least seven of the injured were in critical condition, as they urged people to avoid the area.
French Interior Minister Gerald Darmanin said a fire was underway on Rue Saint-Jacques in Paris' fifth arrondissement, close to the Jardin Du Luxembourg and Sorbonne University.
Firefighters fought a blaze that sent smoke soaring over the domed Pantheon monument and prompted the evacuation of buildings in the neighborhood, police said.
A Paris police official said that the facade of one building had fallen onto the street as a result of the explosion and that many firefighters and members of the police force were attending to the scene.
The blast was followed by a major fire which caused the building, housing a fashion school, to collapse. Images showed wreckage littering the area around the building, as the flames smoldered.
Some 70 fire trucks and 270 firefighters battled the blaze. Nine doctors were also at the scene.
The fire service had said there had been "an explosion" which had "caused the collapse of two buildings," but police later said only one building had collapsed.
Several witnesses told AFP at the scene they had heard "a giant explosion."
Windows as far as 400 meters (440 yards) away were shattered, AFP reporters said.
Paris mayor Anne Hidalgo assembled a crisis unit and wrote on Twitter: "My thoughts go first and foremost to the victims and their loved ones."
The "violent" fire which broke out after the explosion has now been "contained," Paris police chief Laurent Nunez said at the scene, adding that "work is still taking place under the rubble" to find any more possible victims.
The firefighters "prevented the spread of the fire to two adjoining buildings which were seriously destabilized by the explosion" and "were evacuated," Nunez added.
The blast was caused by a "gas explosion," the district's mayor said on Twitter, although this was not confirmed by other officials.
Florence Berthout, mayor of the 5th district in central Paris, said the main building affected is a private fashion school – called Paris American Academy – adjoining the former Val-de-Grace military hospital.
According to the mayor, the noise of the "quite enormous" explosion spread "in part of the district."
An investigation into the causes of the blast was launched immediately, prosecutors said.
Interior Minister Gerald Darmanin cut short a visit to the eastern city of Nancy to head to the scene.
AFP pictures taken at the site showed tall flames and smoke billowing from the building, situated at Place Alphonse-Laveran, close to the Luxembourg Gardens.
The area is at the edge of the Latin Quarter, a top tourism area in the French capital.
A local resident, speaking on condition of anonymity, told AFP that he "heard a huge explosion which made the windows vibrate."
"I thought it was a bombing. It echoed in the apartment. I had 10 seconds of great concern, many people were at the windows," he added.
Another witness, working at the nearby Catholic education secretariat SGEC, said: "There was a big noise. I fell off my chair during a meeting, and so did others."
One of his colleagues had noticed a strong smell of gas in the street just before the explosion, said the man who declined to give his name.
However, officials said they did not have enough elements to determine the cause of the blast with certainty.
Paris prosecutor Laure Beccuau told reporters that "we are obviously counting on the lightly injured people to provide the investigation with input so we can understand what happened."
There have been several incidents of gas-related blasts in the French capital.
The explosion recalled a massive blast that rocked Paris in January 2019, when a suspected leak in a buried gas pipe destroyed a building on the Rue de Trevise in the ninth district, killing four people including two firefighters.
The shockwave blew out scores of nearby windows, and dozens of families were forced to evacuate their homes for months. Much of the street still remains off-limits four years after the disaster.
Paris city hall has been charged with involuntary manslaughter over that blast, and legal wrangling over the exact cause continues.
