One person was still missing on Thursday after a blast ripped through a building in central Paris, while six others were in critical condition receiving urgent treatment in hospital as rescuers sifted through wreckage.

Prosecutors said around 50 people were injured in the explosion and building collapse on the Rue Saint Jacques street, potentially caused by a gas leak.

"Among the two people who were being sought in the rubble, it turned out that one had already been admitted to hospital," the prosecutor's office said.

"The search continues for the second," it added, cautioning: "These figures could still change."

The blast occurred on Wednesday afternoon in the city's fifth arrondissement, close to the Luxembourg Gardens and at the edge of the Latin Quarter, a top tourism area in the French capital.

"It was terrible. I thought it was an earthquake. Everything shook," Violeta Garesteaw, a caretaker in a nearby building, told AFP on Thursday after sweeping up glass in the courtyard.

"A lot of windows were broken. We've had to put up plastic sheets because it's raining," she said.

The shockwave knocked out windows up to 400 metres (440 yards) away.

It sparked a major fire that caused the building -- housing a private fashion school called the Paris American Academy -- to collapse.

According to the mother of one of the students, writing on the school's Facebook account, the classrooms were empty at the time because students were attending a Paris Fashion Week show.

"The toll could have been higher," Paris deputy mayor Emmanuel Gregoire told FranceInfo radio.

Some 70 fire engines and 270 firefighters battled the blaze before it was contained.

On Thursday, the security cordon had been reduced, allowing journalists and gawkers closer to the heap of rubble in front of the structure, just opposite the Val-de-Grace military hospital.

A single fire hose was still sporadically spraying the remains of the building, while some of the nearby shops had reopened.

- 'Strong smell of gas' -


The mayor of the fifth arrondissement said a gas explosion was behind the blast, and several witnesses and resident told AFP they had smelled gas just before it occurred.

But officials said they did not yet have enough evidence to determine the cause of the blast with certainty.

"One of my colleagues noticed a strong gas smell and went outside to look," Philippe Delorme, the head of France's Catholic Education association, whose offices are adjacent to the collapsed building, told RMC radio on Thursday.

"Just when our accountant was dialling the emergency number" of the gas company, "the explosion occurred".

"Obviously we are counting on the victims to give us the first elements for investigating and understanding what happened," Paris Prosecutor Laure Beccuau said at the scene.

The blast also caused extensive structural damage to two adjacent buildings, whose residents had to be evacuated.

There have been several incidents of gas-related blasts in the French capital.

In January 2019, a suspected leak in a buried gas pipe destroyed a building on the Rue de Trevise in the ninth arrondissement, killing four people including two firefighters.

The shock wave blew out scores of nearby windows, and dozens of evacuated families have yet to return to their homes as reconstruction work continues.

Much of the street still remains off limits four years after the disaster, and legal wrangling over the cause of the blast has held up millions of euros in compensation for the victims.