A temperature of 28.8 degrees C (83.8 degrees Fahrenheit) was registered in Paris, surpassing the previous record of 28.4 C measured in October 1921.
In Rennes in the west of France, the mercury rose to 27.4 C, beating the 1946 record of 27.3 C, and in Ploumanach to 30 C from 26 C in 1959, Meteo France official Michel Daloz told AFP.
Across the English Channel, the record for Britain's hottest October day was broken Saturday as temperatures reached 29.9 degrees Celsius in Gravesend, southeastern England, the Met Office said.
The Netherlands also measured its hottest-ever first day of October on Saturday with temperatures reaching 26 degrees Celsius at its central meteorological hub.
"The situation is mainly the result of a high pressure system over eastern Europe from Poland to France, the south of England and the Mediterranean Basin," said Daloz.
The unseasonably warm weather should last until Monday, followed by the first clouds arriving Tuesday from the west.