Traffic monitor Bison Futé reported 250km of traffic jams across France at 9.20am as French holidaymakers joined foreign tourists on the country's main roads.
It took three hours 10 minutes to drive from south Lyon to Orange, compared to the usual one hour 35 minutes, according to motorway operator Vinci, whihc also reported heavy congestion on the A10 towards Bordeaux and between Paris and Poitiers.
Traffic was already heavy earlier in the morning, following Health Minister Agnès Buzyn's appeal to leave as early as 5.00am to reduce congestion and avoid peak temperatures.
Her ministry advised people to stay indoors between the hours of 11.00am and 9.00pm and urged drivers to take regular breaks and drink lots of water.
By midday there were 700km of traffic jams.
Heatwave to continue
Friday night was "very hot" with higher temperatures than Thursday, according to the Météo France weather forecaster, with the therometer not falling below 30.3°C in the south-western city of Perpignan.
A record 67 of mainland France's 95 départements were placed on heatwave alert Friday afternoon and temperatures were expected to rise on Saturday to peaks of 34-36°C in the north of the country and 35-40°C in the south.
They were forecast to continue to go up on Monday and Tuesday but the extreme temperatures are not likely to last as long as 2003's two-week scorcher.
Clear skies for Mars approach
Government spokesman Benjamin Griveaux on Friday denied claims that emergency services were saturated, although Buzyn stressed the need to protect children and the elderly from the effects of the heat.
She was to hold a press conference on Sunday afternoon.
Farming unions expressed concern about the effect of the dry spell on crops.
One of the reactors at France's olders nuclear power station, at Fessenheim in Alsace, stopped production on Saturday, bringing the total number of reactors in France temporarily closed to four.
The measure was taken to avoid pouring too much heated water, used in the productiuon process, into the Canal d'Alsace.
But there was good news for astronomy enthusiasts, after the disappointing cloudy skies during last week lunar eclipse.
This weekend, the 28th "Night of the Stars" organised by the French Astronomy Association, should see clear skies, allowing sky-gazers to see the planet Mars as close to the Earth as it ever gets.