Bahraini authorities say three policemen have been wounded in a petrol bomb attack while securing roads in the capital.

Police said late Saturday that one of the three is in serious condition after they were attacked by a "group armed with Molotov cocktails" as they were trying to extinguish burning tires.

Anti-government activists drawn mainly from Bahrain's Shiite majority frequently set tires alight in protests against the Western-backed Sunni monarchy.

The three policemen were taken to the hospital for treatment. An investigation is underway.