MANAMA, Feb 17 (Reuters) - Bahraini police and anti-government protesters clashed in a number of Shi'ite districts of the Gulf Arab state on Thursday night and the interior ministry said two policemen had been seriously injured in a petrol bomb attack.
Riot police have maintained a heavier presence than usual in areas populated by majority Shi'ites this week to prevent mass protests on the anniversary of the Feb. 14 pro-democracy uprising last year.
In the district of Sar police fired volleys of stun grenades and tear gas to break up groups of teenagers who threw stones and petrol bombs. It was not clear who started the clash. Some residents shouted anti-government slogans and the Muslim call 'Allahu akbar' -- God is great -- from inside their homes or on rooftops.
Police said homemade bombs containing nails had been thrown at them. In the nearby village of Bani Jamra police said they had defused an improvised explosive device left on wasteland. They fired rounds of tear gas into an adjoining cemetery where youths were hiding.
An interior ministry statement said two policemen were seriously injured in a petrol bomb attack on their patrol vehicle on Wednesday night in the town of Sitra. Residents say at least 15 people were later arrested that night.
A medic said at least 120 people were wounded earlier this week. Police have not said how many people they arrested.
Bahrain has been in turmoil since the uprising, mainly by Shi'ites complaining of political and economic marginalisation by the Sunni ruling Al Khalifa family, broke out and was put down by force in March.
The authorities were determined to prevent protesters returning to a symbolically important central roundabout in the capital this week to mark the first anniversary of the uprising.
Riot police and national guard forces maintained a tight cordon of security near the roundabout, which is partly surrounded by barbed wire, and for the first time since a period of martial law last last year deployed armoured patrol vehicles in Shi'ite villages.
The opposition want a shift to fully fledged parliamentary democracy where the elected house forms cabinets, but the government has so far offered only reforms that allow parliament greater powers of scrutiny over ministers and budgets.
(Writing by Andrew Hammond; editing by Tim Pearce)