Wildfires raged across Catalonia and French authorities stepped up restrictions on water use and driving in cities as swaths of western Europe remained in the grip of an intense heat wave on Thursday.

Temperatures climbed toward 44 C in parts of northern Spain and southern France, driving many people to seek relief in the sea, rivers, lakes, fountains and swimming pools.

Spanish firefighters struggled to douse wildfires across more than 6,500 hectares in the northeastern region of Catalonia, and said the affected area could increase as much as fivefold because of the fierce heat and winds.

Helicopters dumped water on the fires, which raged some 80 kilometres inland from Spain's coastal town of Tarragona. There were no reports of casualties, but the regional government said some 45 people had been relocated from farmhouses in the area.

The wildfires are among the worst Catalonia has seen in 20 years, the regional government said, and residents were warned to stay inside to avoid inhaling smoke.

Residents gather to observe a forest fire raging near Maials, Spain, on Thursday. Officials have been warning people to stay inside to avoid inhaling smoke. (Pau Barrena/AFP/Getty Images)

In neighbouring France to the north, authorities extended restrictions on vehicles, which have already been imposed in Paris and Lyon, to Marseille and Strasbourg in an effort to curb air pollution.

Paris banned more than half of the cars — generally older, dirtier models — registered in the capital region, which has nearly six million inhabitants, while the heat wave lasts. Several cities reduced speed limits to reduce car emissions.

Some schools postponed summer examinations, and parts of northern France were put on drought alert, with water supplies to businesses, farmers and ordinary residents restricted. Agriculture Minister Didier Guillaume announced a ban on the transportation of animals until the heat wave has ended.

Electricity in hot demand

Grid operator RTE said French electricity demand on Thursday was close to a summer record seen two years ago, as people turned on fans and coolers to full blast for relief from the scorching temperatures.

"Calls to the emergency services are on the rise nationwide," said Jérôme Salomon, head of national public health. "We are seeing the beginning of a clear impact of the heat wave. For us, the worst is still to come."

He said four drownings directly linked to the heat wave had been recorded in France since the start of the week, as people try to cool themselves down. However, the full toll directly linked to the heat wave will only be known in the days or weeks ahead.

People cool off in the sea in Nice, France, on Thursday, as a heat wave hits much of the country. (Eric Gaillard/Reuters)

French Health Minister Agnès Buzyn said four administrative regions in the south had been placed on red alert, the highest crisis level, with 76 others on orange alert.

The red alert would mean school outings, outdoor sports and other festive activities are suspended or postponed. Buzyn, who described the heat wave as "unprecedented" for France, also cautioned joggers and other sports lovers to curb their activities.

There was at least relief for northern Germany on Thursday as temperatures slid to more normal levels for June. In Berlin, it was 21 C, down from around 37 C on Wednesday.