The Paris prosecutor's office says rescuers have found a body under the rubble of a bakery in Paris that was blown apart by a powerful explosion, bringing the overall death toll to four.
France's Interior Minister Christophe Castaner told reporters at the scene "unfortunately the human toll is particularly serious".
"As firemen were looking for a gas leak in the building, a dramatic explosion took place," Mr Castaner said, adding that one of the firemen had been buried under debris for several hours.
He said 10 people were in critical condition and 37 others were less seriously injured.
He paid homage to the courage of rescuers who saved the life of one firefighter buried for hours.
Spanish Foreign Minister Josep Borrell wrote on his Twitter feed that a Spanish woman had also died in the blast. French authorities did not immediately confirm the third death.
Spanish newspaper El Confidencial reported that the woman was a tourist visiting Paris with her husband, who was unharmed.
Paris Mayor Anne Hidalgo, who was also at the scene, extended a "message of affection and solidarity" to the victims.
Paris prosecutor Remy Heitz said "at this stage we can say it seems from accidental origin, this would be a gas leak".
He said that Paris firefighters were already at the scene for a suspected gas leak on Saturday morning when the explosion happened.
Investigations are ongoing to determine the exact cause of the blast.
Authorities were on high alert in the French capital for a ninth consecutive weekend of "yellow vest" protests, with large parts of the city centre blocked off by riot police.
The force of the blast shattered nearby storefronts and rocked buildings hundreds of metres away, witnesses said.
"The situation is now under control," Mr Castaner said.
He said more than 200 firemen were involved in the rescue operation.
Witnesses described the overwhelming sound of the blast. Charred debris and broken glass covered the pavement around the apartment building housing the bakery, which resembled a blackened carcass.
An eyewitness who was staying at a hotel nearby said he saw a huge fire erupt in the building blown out by the blast.
"There was broken glass everywhere, storefronts were blown out and windows were shattered up to the third and fourth floors," 38-year-old David Bangura said.
He said that as he approached the scene, a woman was crying for help from the first floor of a building: "Help us, help us, we have a child".
More than 200 firefighters joined the rescue operation and two helicopters landed on the nearby Place de l'Opera to evacuate victims.
Ambulances struggled to access the blast area because of police barriers set up to contain any violence by yellow vest protesters.