"In a terrorism-related case, the criminal court sentenced three people to 15 years each in prison, and to have their nationality revoked," Ahmed al-Hammadi, the prosecutor in charge of "terrorist" cases, said in a statement.
He said the three were prosecuted for "possession of explosives" and making home-made devices that were planted to attack security forces, before being discovered and defused.
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Opposition sources in the Shiite-majority but Sunni-ruled kingdom said the three were all Shiites involved in protests.
Since 2001, Bahrain has been rocked by sporadic Shiite-led demonstrations demanding a constitutional monarchy.
Washington ally Bahrain, across the Gulf from Shiite Iran, is the home base of the US Fifth Fleet.