Fernando Alonso took second for Aston Martin, while world championship leader Max Verstappen could only manage sixth as Red Bull struggled at Formula 1's most famous circuit.

The Dutchman's team-mate Sergio Perez was fourth, heightening hopes for Red Bull's rivals of ending the reigning constructors' five-race winning streak to start the season.

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Charles Leclerc, who has taken pole for Ferrari for the last two years at his home race, was fifth.

Mercedes, whose W14 hit the track for the first time with long-awaited upgrades, will be buoyed by Hamilton's third place, but the seven-time world champion's team-mate George Russell struggled to 15th.

As is always the case at the stunning and high-risk street circuit, there was drama and incident, with several drivers escaping contact with the barriers before crashes for Nico Hulkenberg and Alex Albon resulted in red flags.

The latter incident, a heavy shunt for Williams' Albon at the opening corner Sainte Devote, saw the session come to a premature end with three minutes left on the clock when it occurred.

'Red Bull have serious competition this weekend'

After the cancellation of last weekend's Emilia-Romagna Grand Prix due to severe flooding in northern Italy, there was even greater intrigue than usual as Formula 1 returned to Monaco.

Along with the usual glitz and glamour that Monaco brings to the calendar, there was mystery surrounding Mercedes' upgrades, and excitement that Ferrari and Aston Martin may be able to outpace Red Bull for the first time this season.

The last element, perhaps the most crucial to ensuring that excitement remains throughout the weekend, looks to be in play, with Sainz and Alonso showing impressive early pace.

Ferrari have shown impressive form at Monaco in recent years, and Sainz's 1:13.372, which was more than 0.3s clear of Alonso in second, suggests they will be a major threat for pole position on Saturday.

Alonso, who has secured four podiums in his first five races with Aston Martin, has been open about his belief that this weekend he can claim a first F1 victory in more than a decade, and will be encouraged by a solid opening 60 minutes.

After a dominant start to the season which has seen him win three races and amass a 14-point world championship lead over Perez, Verstappen cut an extremely frustrated figure as he struggled from the off and demanded early set-up changes.

He reported that his car felt "a little better" afterwards, but was warned over radio that he would have to "live with" the discomfort for the remainder of the session, and was visibly irritated as he failed to make a chicane.

"Red Bull will home in on where they need to be," Sky Sports F1's Martin Brundle said. "The story of FP1 is that Red Bull have got some serious competition this weekend."

The constant danger of the Circuit de Monaco was highlighted as Hulkenberg lost control of his Haas at the Nouvelle Chicane, spinning, hitting the barrier and suffering a puncture.

The German was able to limp back to the pits but debris left on track led to a brief red flag stoppage to allow the track to be cleared.

Mercedes finally unveil W14 upgrades

All eyes had been on the Mercedes garage since the team began setting up on Thursday morning, with the Silver Arrows finally moving on from the 'zero-sidepod' concept they have pursued since the start of last season.

Photos of the new bodywork, which is accompanied by a new floor and front suspension, emerged on Thursday, but the car was finally revealed in full form shortly before practice began on Friday.

Initial assessments were made, but ultimately how the W14 performed on track is what team, drivers, and fans were waiting to see.

The results were somewhat confusing, with Hamilton settling quickly and sitting at the top of the timesheet for a period of the session before finishing third.

In contrast, Russell was uncomfortable from the start, and twice returned to the garage for significant set-up changes.

Despite the changes, he remained uncomfortable, explaining over radio that he was lacking "confidence".

Mercedes trackside engineering director Andrew Shovlin explained to Sky Sports F1 after the session that the British duo had been running different set-ups.

"We haven't seen major problems which is a useful start to it," he said. "We were doing some comparisons on set-up across the cars so I think we have a good direction there - Lewis appeared to be in a happier place than George.

"One of the tricky bits with Monaco is the track evolves so much. It's one thing having the car in a good spot in FP1, you need to keep it there in FP2, FP3 and into Qualy. That's a lot easier said than done. It's a good start but we're certainly wary of the many ways you can get Monaco wrong."