Police in Bahrain said on Wednesday they uncovered a weapons cache inside an unlicensed mosque where locally-made bombs were being produced.
Authorities said they seized electric detonators, stopwatches and other materials used to make bombs.
The cache was uncovered in the area of Abou Baham, a scene of violence attributed by official media to Shiite hardliners.
On Sunday authorities detained seven men over the killing of a policeman, as demonstrators tried to break through police checkpoints around the village where he lost his life.
Bahrain, which hosts the U.S. Fifth Fleet, has been convulsed by unrest since February last year following mass demonstrations led by majority Shi’ites demanding democratic change in the Sunni-led monarchy.
The ruling Al Khalifa family brought in Gulf Arab troops, mainly from Saudi Arabia, and imposed over two months of martial law to end the uprising.
The incident in the early hours of Friday was the first in which a policeman had been killed since martial law ended in June 2011. Policemen were attacked by rioters with petrol bombs and an unspecified "explosive device", the authorities said.
"Seven Bahrainis have been detained and have been referred to the public prosecution in the case of a bombing attack in al-Eker ... in which one policeman was killed and a second critically wounded during a routine patrol," a statement from the government’s Information Affairs Authority said.
It named the slain policeman as 19-year-old Imran Ahmed but did not give his nationality. Many Pakistanis and some Arab nationals serve in Bahrain’s riot police - a source of friction with protesters.
The opposition, which says more than 45 people have died in clashes since martial law ended, want full legislative powers for parliament and for the makeup of the Cabinet to be approved by parliament too. The Cabinet has been headed by an uncle of King Hamad bin Isa since 1971.