On the time sheet, Leclerc leads by a very short head (39/1000) the Red Bull of the Mexican Sergio Pérez and by 70/1000 the single-seater of his teammate, the Spaniard Carlos Sainz.
Behind the trio, the Dutchman Max Verstappen (Red Bull) - current leader of the championship - sets the fourth time, 181/1000 from his rival in the championship.
The McLaren of Briton Lando Norris 5th and the AlphaTauri of Frenchman Pierre Gasly, 6th, follow.
The other Frenchman in the event Esteban Ocon (Alpine) posted a meager 16th time.
At Mercedes, in great shape during the previous round in Spain, the Britons George Russell and Lewis Hamilton took the 8th and 10th times respectively.
This first test session in Monaco, a historic F1 event, was marked by a single red flag.
Victim of a mechanical problem, the German Mick Schumacher (Haas) stopped near the track, blocking the entrance to the pit lane.
1st free practice session:
Charles Leclerc (MON/Ferrari) 1:14.531 (29 laps)
Sergio Perez (MEX/Red Bull) 1:14.570 (30)
Carlos Sainz Jr (ESP/Ferrari) 1:14.601 (28)
Max Verstappen (NED/Red Bull) 1:14.712 (26)
Lando Norris (GBR/McLaren-Mercedes) 1:15.056 (31)
Pierre Gasly (FRA/AlphaTauri-Red Bull) 1:15.083 (33)
Daniel Ricciardo (AUS/McLaren-Mercedes) 1:15.157 (33)
George Russell (GBR/Mercedes) 1:15.211 (34)
Sebastian Vettel (GER/Aston Martin-Mercedes) 1:15.387 (33)
Lewis Hamilton (GBR/Mercedes) 1:15.499 (29)
Yuki Tsunoda (JPN/AlphaTauri-Red Bull) 1:15.536 (36)
Lance Stroll (CAN/Aston Martin-Mercedes) 1:15.539 (26)
Fernando Alonso (ESP/Alpine-Renault) 1:15.749 (28)
Kevin Magnussen (DEN/Haas-Ferrari) 1:15.806 (30)
Alexander Albon (THA/Williams-Mercedes) 1:16.110 (39)
Esteban Ocon (FRA/Alpine-Renault) 1:16.315 (24)
Zhou Guanyu (CHN/Alfa Romeo) 1:16.417 (27)
Nicholas Latifi (CAN/Williams-Mercedes) 1:17.714 (30)
Mick Schumacher (GER/Haas-Ferrari) 1:18.636 (12)
Valtteri Bottas (FIN/Alfa Romeo) 0.000 (2)
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