Headaches and nausea were among the complaints in calls made overnight to emergency lines in Paris by more than 10,000 people worried by the stench of rotten eggs that had invaded their streets and homes.
But France's Ecology Minister Delphine Batho, who cut short an official trip to Berlin to rush to the site of the leak at a chemical plant in the picturesque city of Rouen in Normandy, said there was no health risk.
The leak began early Monday at a Lubrizol plant.
Winds carried the foul-smelling invisible gas down the densely-populated Seine river valley to Paris, and later northwards over the Channel and into England, where it even reached south London.
"South Kent residents are being asked to keep doors and windows closed due to a gas cloud that is believed to have come across from France," the fire and
rescue service in the southeastern English region said.
The offending odour came from a gas called mercaptan, which, among other uses, is added to municipal gas because its sulphurous smell alerts people to gas leaks.
The Lubrizol plant, which makes additives for industrial lubricants and paint, shut down production as they battled to plug the leak which company executives hoped would be done later Tuesday.
Regional authorities ordered the postponement of a French Cup tie match in Rouen between the city's football team and Marseille on Tuesday evening.
"We didn't want to be in a situation where we have 10,000 spectators two kilometres away from the plant without any capacity for confining or evacuating them if that were necessary," said senior local official Florence Gouache.
Snow had already threatened the game — a sellout — although a pitch inspection on Monday had led to the match being given the go-ahead prior to the gas leak.
Despite the official insistence that there was no danger, French social media were awash with people in the affected regions complaining of headaches and nausea from the gas that smelled like rotten eggs.
"They're all saying not to panic, but they said the same thing about the cloud from Chernobyl," said mother-of-four Patricia Cousteau, referring to radioactive fallout that spread across Europe in 1986 after an explosion at a Ukrainian nuclear plant.
Authorities said in an earlier statement that a chemical substance at the Lubrizol plant became unstable and caused odours that are similar to those of town gas."The gas has an unpleasant smell but is not toxic," it said.
The concentration of the gas was also "very low", the statement said, adding that "a large number of people have been inconvenienced".