France's ecology minister said Tuesday she was rushing back home from a trip to Germany after a gas leak at a chemical plant spread fumes over Paris and forced the cancellation of a major football match.

Authorities insisted the gas, mercaptan, was harmless, but emergency lines were inundated with calls from people worried about the smell that came from a Lubrizol factory in the Normandy city of Rouen, northwest of the capital.

Ecology Minister Delphine Batho said in a statement she was heading for Rouen to oversee operations to ensure that the leak was being dealt with safely.

Winds carried the smell 100 kilometres (60 miles) down the Seine valley from Rouen to the capital, where more than 10,000 people phoned fire and ambulance services overnight to complain about the stench.

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.

Regional authorities ordered the postponement of a French Cup tie match in Rouen between the city's football team and Marseille on Tuesday evening, the host club revealed.

"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.

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".

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