A bomb attack carried out by "terrorists" wounded four Bahraini policemen in a Shiite-populated village overnight, the official BNA news agency said on Wednesday.
"A terrorist explosion targeted security forces that were escorting firefighters... on their way to the scene" of a blaze in the village of Diraz, BNA reported, citing a senior police official.
The official said "four policemen were wounded" in the explosion, adding that "two of the officers suffered serious injuries".
Witnesses at the scene said security forces imposed a total lockdown on Diraz following the explosion, shutting off all routes out of the village.
Police also raided 11 homes and arrested six people, the witnesses added.
The exact cause of Tuesday night's blast was unknown, but "an act of sabotage" sparked the fire at a shop, the official said, adding an investigation was launched into the "terrorist act".
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Tensions and violence have been escalating in the Sunni-ruled kingdom as Shiite-led protests calling for the release of prominent activist and hunger striker Abdulhadi al-Khawaja intensify amid fears he may die in prison.
Khawaja's health has drastically deteriorated since he started refusing food on February 8, relatives and rights groups say. A Shiite, he is one of several democracy activists sentenced to life in prison after being convicted of conspiring to overthrow the monarchy.
The Gulf state's Shiite majority claim marginalisation and disenfranchisement by the Sunni regime, and for months have been calling for political and social reforms.
The explosion came a day after riot police fired tear gas to disperse protesters as they approached Pearl Square, the epicentre of last year's Shiite-led uprising against the regime.
The small group of protesters had broken off from a larger rally in the Shiite village of Jidhafs, where hundreds marched in support of Khawaja, demanding his release.
Tuesday's protests followed clashes a day earlier between police and protesters during the funeral procession of 36-year-old Salah Abbas Habib, who was found dead in a Shiite village over the weekend.
Habib's death coincided with the controversial staging of the Bahrain Formula One Grand Prix a year after the event was cancelled following the March 2011 crackdown on the month-long uprising, which according to an independent probe, left more than 35 people dead, most of them Shiites.