Fans hummed in every corner, and sheets hung from windows at an overcrowded prison near the city of Toulouse, where inmates tried to cool down as temperatures pushed past 36 degrees Celsius, an AFP journalist observed on Thursday.

Southern France is in the grip of a heatwave, and for inmates at the Seysses detention centre, that heat is "unbearable", one detainee said.

But like many prisons in France, Seysses is overcrowded - in June, it held on average more than twice its capacity.

"Having two people per cell has become the exception," said one guard, whose name like all the detainees interviewed cannot be published.

With detention centres across the country at over capacity and facilities ill-suited to ever more frequent heatwaves, guards and inmates alike worry about the impact of the heat.

Heat can create security risks, with fights becoming "more frequent" as temperatures rise, the guard said.

"The slightest annoyance can spark a fight."

Temperatures this weekend are expected to hit 40C in some areas and Monday forecast to be the "hottest day nationwide," according to national weather service Meteo France.

At the prison, "you can really feel the 38 degrees. The walls absorb the heat. There's condensation," the detainee said, speaking from the 9-square-metre cell he shares with two other people.

"We're not going to ask for air conditioning, that's not going to happen, but we would like more ventilation or a larger cell," he said.

-'No choice'-

As the planet warms as a result of humanity's emissions of greenhouse gases, experts say heatwaves are likely to become more frequent across Europe.

For detainees at Seysses, a two-hour break in an asphalt courtyard with no trees offers little reprieve, with mist sprayers not due to be installed until next summer.

An inmate in the women's section of the prison said she and her fellow prisoners just stayed in their cells because it was too hot to go outside.

For the inmates who did venture into the recreation yard, most stayed in the shade, though a few did some push-ups or pull-ups even in the blistering heat.

Prison staff distributed free water and provided fans and hats to the most vulnerable individuals, including those over 75 years old or those with health issues.

But the prison's facilities, including a poorly ventilated library and a gym, with tiny windows and a single fan, offer little escape from the heat.

"It's hot in there, but when there are a lot of them, it gets very, very hot very quickly," said a guard, describing the workout room.

Even sleep brings little relief, said one recent arrival in the women's section of the prison, which is built for 40 people but holding 80.

The inmate, who shares her cell with two other detainees, said she has no alternative but to sleep on a mattress on the floor.

Sleeping is "difficult, because it's concrete and during the summer, it's quite hot," she said.

"But we have no choice."