Australian Daniel Ricciardo set the pace for Red Bull in Monaco Grand Prix practice on Friday, with crashes and a loose drain cover putting safety back in the spotlight at Formula One's showcase race.
Ricciardo lapped the twisty harbourside circuit in a quickest time of one minute 14.607 seconds, 0.606 faster than triple champion Lewis Hamilton in the closest Mercedes with championship leader Nico Rosberg third.
Hamilton was fastest in the morning with a lap of 1:15.537 on the new ultrasoft Pirelli tyres making their debut this week.
The Briton, who collided with Rosberg in Spain 11 days ago and lags the German by 43 points after five races, is seeking to end his team mate's run of three successive Monaco wins but Ricciardo could be a threat to both.
Red Bull had targeted Monaco as their best chance of victory this season, before Dutch teenager Max Verstappen won in Spain on his debut for the team.
Ricciardo has a new specification Renault engine for Sunday's race, which Verstappen will not get until the next race in Canada due to a shortage of parts.
Verstappen was fifth and fourth in the respective sessions.
Ferrari's Sebastian Vettel, who spun at Mirabeau and hit the barrier after lunch, had been third fastest in the morning but four tenths slower than Hamilton.
With the paddock rocked by news that the family of the late French driver Jules Bianchi were taking legal action against Formula One, his former Marussia team and the governing FIA after his death last year, safety was a talking point.
A series of crashes saw the virtual safety car deployed three times in the morning before the session was stopped when a loose drain cover damaged Rosberg's car and Jenson Button's McLaren.
Photographs showed a sizeable piece of metal had been dislodged, fortunately without consequences for Button or anyone following behind.
Track workers were deployed to Sainte Devote to weld down the offending cover.
Brazilian Felipe Massa, who was hit on the head and seriously injured by bouncing debris at the 2008 Hungarian Grand Prix, had crashed out earlier when he skewed his Williams into the barriers at the first Sainte Devote corner.
Mexican Esteban Gutierrez stopped his Haas before Portier with electrical problems, and then Renault's British rookie Jolyon Palmer crashed at Tabac.
"Sorry guys," Palmer, whose Danish team mate Kevin Magnussen also crashed in the afternoon, said over the radio.
Yellow warning flags were also waved when Brazilian Felipe Nasr's Sauber went off at Sainte Devote.
Indonesian rookie Rio Haryanto crashed his Manor in the second session at the tunnel exit, coming to rest unhurt in the escape road.