Seven-time world champion Lewis Hamilton topped the times ahead of Australian Oscar Piastri in an intriguing opening free practice for this weekend's Monaco Grand Prix.
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Hamilton clocked a best lap in his Mercedes of 1minute and 12.169 seconds to outpace McLaren's Piastri by 0.029sec with his 'silver arrows' teammate George Russell third.
Lando Norris was fourth in the second McLaren ahead of local favourite Charles Leclerc in the leading Ferrari, two-time champion Fernando Alonso and his Aston Martin teammate Lance Stroll.
The late pause, after Chinese driver Zhou Guanyu lost control of his Sauber, interrupted some teams' plans to switch to soft tyres in the closing minutes, leading to an unrepresentative outcome.
Three-time world champion Max Verstappen, who did not run on softs, complained about his Red Bull car as he struggled in 11th place and 0.815secs off the pace.
"It is so loose on the bumps, any correction with engine braking or brake bias just makes it worse," said Verstappen, who remarked on Thursday he envisaged a tough weekend labelling the track "one of the more difficult" for the team.
McLaren introduced a striking yellow and green livery as a tribute to the late Ayrton Senna, one of many changes as the teams switched from the sweeping corners of Imola last weekend to the narrow and unforgiving barriers of Monaco.
On an overcast day with rain threatening, Hamilton led the cars out of the pit-lane to start the session, most teams opting for hards.
Piastri set the early pace ahead of Verstappen whose father Jos was present for the first time since Red Bull's early season off the track difficulties.
Norris soon took over for McLaren with a lap in 1:15.294 before Leclerc gained the initiative in 1:14.238.
Norris and Leclerc continued swapping fastest laps, the Monegasque showing his form on the track that was his childhood bus-run to school.
After 30 minutes of the hour Verstappen and then Hamilton moved up to second before Russell went top on softs ahead of his Mercedes teammate.
Hamilton then improved to 1:12.169 to go top before Zhou Guanyu crashed to trigger a red flag.
The Chinese driver clipped a barrier and lost part of his front wing which was then collected by Leclerc's Ferrari as he ran over the debris.
This forced Leclerc to pit for repairs as light rain fell and on resumption Perez pitted with a puncture.