PARIS Six people remained in a critical condition and one person was believed still missing on Thursday, one day after a blast ripped through a street near Paris' historic Latin Quarter, the city's public prosecution office said.

"These figures may still change," prosecutor Maylis De Roeck told Reuters in a text message, adding that around 50 people had been injured in the blast, which set buildings ablaze and caused the front of one to collapse onto the street.

Of two people initially believed missing, one has been found in hospital and is being taken care of, the prosecutor said, adding: "Searches are ongoing to find the second person."

Authorities have not yet said what caused the explosion, which witnesses said had followed a strong smell of gas at the site.

The explosion led to scenes of chaos and destruction in the historic Rue Saint Jacques, which runs from the Notre-Dame de Paris Cathedral to the Sorbonne University, just as people were heading home from work.

It also destroyed the facade of a building housing the Paris American Academy design school popular with foreign students.

Florence Berthout, mayor of the Paris district where the blast occurred, said 12 students who should have been in the academy's classrooms at the time had fortunately gone to visit an exhibition with their teacher.

"Otherwise the (death toll) could have been absolutely horrific," Berthout told BFM TV.

She said three children who had been passing by at the time were among the injured, although their lives were not in danger.

(Reporting by Bertrand Boucey, Benoit Van Overstraeten, editing by Tassilo Hummel and Gareth Jones)