"The durable heatwave episode which has started this Tuesday afternoon will last at least until Friday 26 August," the official Météo France weather service announced.
On Tuesday it put 22 départements, including the Paris region, on orange alert, reducing the number to 11 on Wednesday morning then raising it to 37 in the afternoon.
The majority of the rest of the country was on the the next highest level, yellow.
Temperatures will range from 18-20°C to 35-38°C and will need particular attention in urban areas where they are not likely to fall below 20°C.
The highest temperatures at 4.00pm on Tuesday were recorded in the western - Rennes (34°C), Nantes, Cognac, Bordeaux (35°C) and Mont-de-Marsan (36°C).
The départements on orange alert are mostly in the north and centre of the country, not because temperatures will be higher there but because of the length of time the temperature remained high and because the population in the south is more used to high temperatures.
Officials in the Ile de France region around Paris raised the alert to level three, while the Paris city council set up an emergency hotline and opened "cool rooms" in retirement homes and council premises.
A heatwave is declared if high temperatures are recorded both day and night for three days in a row.
"The hotter it is in the day, the more tired we get," Météo france explains. "The hotter it is at night, the less one recovers."
This is the second heatwave this year, following a similar episode in July.
Some 15,000 people, most of them elderly, died in a heatwave in France in 2003, leading to a revision of procedures and increased vigilance by the authorities.