• Championship leader Max Verstappen was struggling in sixth in his Red Bull.
• Albon's crash prevented Leclerc from setting a lap that might have threatened Sainz.
Williams driver Alex Albon brought the first practice session at the Monaco Grand Prix to an early end with a crash at the first corner.
The British-born Thai, who had impressed with 10th the fastest time up to then, lost the rear mid-way through Saint Devote and hit the barrier.
Ferrari's Carlos Sainz set the pace from Aston Martin's Fernando Alonso and Lewis Hamilton in an upgraded Mercedes.
Championship leader Max Verstappen was struggling in sixth in his Red Bull.
Verstappen, complaining that the car was set too low and bottoming, was behind his team-mate Sergio Perez and Ferrari's Charles Leclerc.
Albon's crash prevented Leclerc from setting a lap that might have threatened Sainz.
Leclerc, born and raised in Monaco and on pole here for the last two years, had been forced to abandon a quick lap which would have been slightly slower than Sainz's when he had a moment at the second part of the Swimming Pool and was poised to try again when the session was stopped because Albon's car was in the middle of the road at the exit of Sainte Devote.
Alonso, who was 0.338 seconds slower than Sainz, was also on a quicker lap when the session stopped with a little over three minutes to go.
George Russell was also struggling in the second Mercedes. He was 15th fastest and a second slower than Hamilton and complaining of a lack of rear grip and being unable to get his tyres up to temperature.
Hamilton had an encouraging session in the Mercedes, which has a major redesign featuring revised front suspension, sidepods and floor as they seek to begin a route back to competitiveness.
Hamilton was close to the front throughout and was briefly fastest before being surpassed by Sainz and Alonso.
Nico Hulkenberg was another driver to fall foul of the Monaco walls, breaking his wheel on the barrier on the inside of the harbour-front chicane.
Lando Norris impressed in the McLaren, setting seventh fastest time in a car that was among the slowest of all at the last race in Monaco.