Ahead of today's opening session, the air temperature is 20 degrees C, while the track temperature is 28 degrees. It is bright and sunny.

Other than the weather - rain is expected at some point this weekend - the big concern here is traffic, with four cars more than we're grown used to in recent years.

Force India is keen to see both its drivers get maximum running today therefore Paul di Resta will watch from the pit-wall. Same goes for Christian Klien at Hispania.

Tyre options this weekend are medium (prime) and supersoft (option). McLaren noticeably running new wheel rims after it was confirmed that a stone getting into the wheel rim was the cause of last week's off in Spain.

Liuzzi first out, followed by de la Rosa, di Grassi, Kovalainen, Buemi, Glock, Alguersuari, Vettel, Hulkenberg and a whole load of others as the Principality gets a rude wake up call.

Five minutes into the session all but Hamilton and Chandhok have been out, the Englishman joint favourite here this weekend along with Sebastian Vettel.

As Chandhok and the Hamilton finally leave the pits, we are treated to some superb super-slow shots of Massa and Button riding the kerbs, the sort of thing we normally only get in Germany.

Fifteen minutes into the session, Alguersuari is the first driver to come out for a second run. Other than a little bit of silliness, for which he was rightly penalised, the Spaniard had a great race last weekend.

The Toro Rosso driver crosses the line at 1:30.020 having noticeably lifted off in the Tunnel. Next time around the youngster posts 26.141.

As Alguersuari raises the benchmark with a 23.370, Vitaly Petrov heads down the pitlane.

More and more drivers pour onto the track, among them Jenson Button, as Alguersuari posts 21.225 and Petrov 25.601.

Button's first flying lap sees the McLaren driver go second (23.667), ahead of Petrov and Hulkenberg. Even with just 10 cars on track Button found himself somewhat hampered.

A 20.564 sees Button go quickest as Kobayashi goes fifth ahead of Trulli, de la Rosa and Senna.

Alonso, Massa and Hamilton leave the pits as the session reaches one-third distance. Meanwhile, Alguersuari, who has completed 12 laps, retakes the top spot with a 20.190, only to lose it again when Button improves with an 18.468.

Buemi goes second (19.895) and Massa fourth (40.471) as Hamilton crosses the line at 26.603.

Button improves with a 17.618 as Massa goes second and Hamilton third. Schumacher posts 20.346 to go eighth.

As Hamilton goes quickest in the middle sector, Kubica goes fifth overall. No sign of the Red Bulls as yet, while Rosberg goes fourth with an 18.403. Hamilton posts 17.423 to make it a McLaren 1-2.

Massa splits the McLarens with a 17.471 as Schumacher improves to fourth with a 17.941 only to be demoted by Alonso who posts 17.482.

The pace is fast and furious as Massa goes quickest (16.997) only to be almost instantly leapfrogged by Hamilton who crosses the line at 16.963. Moments later the Brazilian posts 16.476.

Kubica goes third with a 17.106 as Liuzzi, Sutil, Vettel, Webber and Chandhok remain the only drivers yet to post a time.

Replay shows Alonso lightly brushing the barrier at the exit from Rascasse with the rear of his Ferrari.

At half-time, it's: Massa, Hamilton, Kubica, Rosberg, Alonso, Buemi, Schumacher, Button, Barrichello and Kobayashi.

Webber and Vettel finally leave the pits, the Australian goes fifteenth with a 19.722 while his German teammate posts 22.182. While Vettel subsequently improves with an 18.582, Webber remains on 17.22 before improving with an 18.218 which puts him twelfth.

Chandhok gets all out of shape at the entry to Casino Square, loses the back end lightly clouts the barrier with the rear of his car.