Leclerc fastest in first French practice ahead of F1 championship leader Verstappen
The Monegasque, the most local of any of the drivers who will be competing in Sunday's race and winner of the previous round in Austria, lapped with a best time of one minute 33.930 seconds on the soft tyres.
Red Bull's championship leader Max Verstappen (24) was second and 0.091 slower, despite running slightly wide on his best lap, also on softs on a hot afternoon in southern France.
Leclerc did 23 laps to Verstappen's 19.
Ferrari's Carlos Sainz (27), winner of this month's British Grand Prix, was third on the timesheets but the Spaniard will have a grid penalty of at least 10 places after Ferrari changed his car's control electronics.
George Russell (24) was fourth for Mercedes, with seven times world champion team mate Lewis Hamilton (37) sitting out an opening session for the first time to make way for reserve driver and Formula E champion Nyck de Vries (27).
All teams are obliged under new rules to run drivers with limited Formula One experience in two Friday free practice sessions this season.
De Vries ended the session with the ninth fastest time.
Team boss Toto Wolff (50) said Hamilton, watching the new occupant of his car from the pitwall ahead of his 300th race, had been helping de Vries.
Frenchman Pierre Gasly (26) was fifth, ahead of Red Bull's Mexican Sergio Perez (32) who spun at turn three but managed to keep his car out of the barriers.
McLaren's Lando Norris (22) was seventh fastest and again outperformed his Australian team mate Daniel Ricciardo (33) in 10th place, with the team bringing plenty of new parts to the Paul Ricard circuit.
Polish reserve Robert Kubica (37) replaced Valtteri Bottas (32) at Alfa Romeo for the session and was 19th of 20.