Weather agency Météo France has issued weather warnings for most of France, with the number of départements placed on orange alert rising to 43.
The alerts, sent out in a bulletin on Thursday afternoon, are in place until Saturday morning.
The départements where the alert was raised from yellow to orange were: Jura, Meurthe-et-Moselle, Moselle, Haute-Saône, Vosges and Territoire de Belfort.
On Friday, temperatures in the east of the country were expected to rise to 35-37C, while seeing a slight drop in western areas.
The agency predicted a further rise in the east on Saturday, with Paris and central France likely to reach a maximum of 35-38C.
The agency said that even healthy people will be at risk if they're in the areas with orange alerts, and it urged people to watch out for signs of heat stroke, especially among the sick and elderly.