Defending double world drivers' champion Fernando Alonso topped the times in Thursday's opening free practice session for Sunday's Monaco Grand Prix.
The 25-year-old Spaniard led a McLaren Mercedes-Benz one-two ahead of his team-mate, 22-year-old British rookie Lewis Hamiloton, the youngest leader of the championship in Formula One history.
The two Ferraris of Finn Kimi Raikkonen and Brazilian Felipe Massa wound up in fifth and eight places respectively after a typically incident-filled first session on the unforgiving Mediterranean street circuit.
Hamilton set the early pace and his lap in one minute and 18.686 seconds after 24 minutes was 1.5 seconds faster than Alonso's first flying lap on a circuit where the young Briton has won all three of his previous races in junior formula races.
But Hamilton's luck ran out after 14 laps on the tight and twisty circuit when he was brought to a halt by a broken starter motor on his car. The team said a piece of debris had been left in the gearbox forcing mechanics to remove the floor of the car to fix the problem.
Hamilton's advantage was soon cut to 0.4 seconds when the session resumed after a red flag interruption caused when Dutchman Christijan Albers' stationary Spyker car was retrieved from Tabac corner.
Massa, concerned by the uncertainty ahead in a race weekend that he has described as a lottery, then took over at the top of the time sheets before Hamilton responded with a lap in 1:17.601.
Despite both Massa and Alonso clocking improved times, Hamilton managed to stay six-tenths of a second ahead until Alonso used a set of super-soft tyres on the hour to clock 1:16.973.
That put him six-tenths clear of the field and enabled him to stay there. The leading positions remained unchanged in the final half hour as the Ferraris fell off the pace while German Nick Heidfeld, in a BMW Sauber, recovered from an excursion across the chicane to improve and take third spot ahead of Italian Giancarlo Fisichella in a Renault.
Fisichella's efforts ended prematurely when he went into the barriers and touched an inside kerb at Sainte Devote.