By Alan Baldwin

MANAMA (Reuters) - Nico Rosberg wrapped up practice for the troubled Bahrain Formula One Grand Prix by putting Mercedes on top of the timesheets in a dusty final session at the Sakhir circuit on Saturday.

While security was tight on the roads leading south from the capital Manama after another night of clashes between teargas-firing police and petrol bomb-throwing protesters, it was business as usual in the paddock as teams prepared for qualifying.

German Rosberg, who suffered a bird strike on his helmet in Friday's practice when he was again the fastest man on track, lapped the reconfigured 5.412-kilometres layout with a best time of one minute, 33.254 seconds.

In dusty and gusty conditions, with sandstorms blowing in from nearby Saudi Arabia to the west, Red Bull's double champion Sebastian Vettel was 0.147 seconds slower than his compatriot.

The Ferrari pair of Spain's Fernando Alonso and Brazilian Felipe Massa were 10th and 16th respectively.

The team decided on Friday to pack up early to avoid having to travel after nightfall, despite Formula One supremo Bernie Ecclestone's offer to go with the mechanics at any time of the evening and without any official escort.

Force India's British driver Paul Di Resta was 15th, after lapping third fastest on Friday morning, and German team mate Nico Hulkenberg 19th.

Venezuelan Pastor Maldonado's Williams had an unscheduled gearbox change, meaning that he will take a five-place grid penalty after qualifying.

The paddock, seemingly secure behind its electronic turnstiles and metal fences with teams housed in solid buildings, remained an oasis of calm in contrast to other parts of the island.

An airport-style scanner has been installed at the circuit entrance used by the media, with one reporter surprised on Saturday to have a plastic bottle of water confiscated.

International Automobile Federation (FIA) President Jean Todt also made an appearance in the paddock after a visit on Friday by Bahrain Crown Prince Salman.

(Editing by Peter Rutherford)