Lawyer Manar Makki said another nine people in the case were sentenced to six years imprisonment. All have the right to appeal.
The policeman had been killed by a homemade bomb.
Anti-government activists, primarily drawn from the Arab Gulf monarchy's Shiite majority, frequently clash with police, and hard-line protesters have at times used improvised explosives. This month, a policeman and a civilian were killed in separate explosions. Three policemen were wounded in a third blast.
His lawyer said Salman has been accused of inciting hatred against the Sunni-led monarchy and calling for its overthrow by force.
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Salman, the leader of the main opposition group al-Wefaq, was transferred to public prosecutors where he could face formal charges.
Al-Wefaq boycotted last month's parliamentary elections.
The group is calling for greater power-sharing between elected lawmakers and the monarchy, the release of political prisoners and a prime minister who is chosen by elected officials and not the king.
"Resorting to such policies merely lead to further complication of the conditions," Afkham said in remarks carried by Iran's semi-official Fars news agency.
She urged Bahraini officials to hold serious national dialogue talks between the government and political factions. Bahrain and its Sunni allies, primarily Saudi Arabia, have accused Iran of meddling in the region's internal affairs.
Gulf Arab countries sent regional forces to Bahrain in 2011 at the height of protests to help quell the Shiite uprising.