After five rounds of Mercedes dominance, the return to Europe on the twisty principality street circuit has been tipped as an opportunity for rival teams to level the playing field.
The early signs were encouraging as local favourite Leclerc, winner in 2024 and runner-up last year, delighted the home fans with a quickest lap of one minute 13.978.
Three-time Monaco winner Hamilton, still hunting his first victory since joining Ferrari before the start of last season, embraced the glamour of the Monaco race with a dazzling sparkly pink helmet and went second, 0.226 seconds behind.
Monaco's relatively slow corners and lack of long straights were expected to negate the power advantage enjoyed by Mercedes, who have won all five races this season.
Championship leader Kimi Antonelli was fourth quickest, 0.559 seconds slower, with Mercedes teammate George Russell fifth, 1.005 seconds back. Red Bull's Max Verstappen was third, 0.513 seconds behind, while last year's winner and reigning world champion Lando Norris was sixth fastest.
The opening skirmishes of the iconic race provided plenty of incident in warm and sunny conditions as drivers struggled for grip on the bumpy surface, shaving the barriers by centimetres.
The session was twice interrupted by red flags, the first after Red Bull's Isack Hadjar slammed into the safety fencing on the exit of the swimming pool chicane, walking away unhurt.
Aston Martin's Fernando Alonso also went off near the end of the session, damaging the front end of his car.