Three other Shiite activists, who were also on trial, were sentenced to life in prison for their role in the police officers' deaths.
The ruling came as tensions between Bahrain's Sunni Muslim ruling family and its Shiite majority continued to rise in the aftermath of pro-democracy protests that the kingdom crushed last month with military help from its Gulf Arab neighbors.
Government officials have said that a total of four policemen were killed during the unrest that gripped the country in February and March, at least three of whom were run over by cars around March 16.
The verdicts were the first related to Bahrain's uprising, which was inspired by revolts in the Arab world.
The kingdom's Shiite majority has long complained of discrimination and is campaigning for greater freedoms and equal rights in the tiny Sunni-ruled island nation.
Faced with an unprecedented political unrest, Bahrain's king declared martial law and invited troops from Saudi Arabia and other Sunni-ruled Gulf countries to help quell dissent after weeks of street marches and sit-ins in the kingdom's capital, Manama.
The seven opposition supporters sentenced Thursday were tried behind closed doors on charges of premeditated murder of government employees.
In an earlier hearing this week, Bahrain state media said the military prosecutor presented evidence that showed the defendants killed the policemen "on purpose" by running them over with a car.
Their lawyers denied the charges.
Hundreds detained
Hundreds of protesters, opposition leaders and human rights activists have been detained since emergency rule was declared March 15.
Earlier this month, the authorities banned media from covering legal proceedings in the country's military courts.
Among those detained are also dozens of Shiite professionals, such as doctors and lawyers, including a lawyer who was to defend some of the seven opposition supporters in the military court.
The attorney, Mohammed al-Tajer, is one of Bahrain's most prominent human rights lawyers.
He has represented hundreds of clients against the state, including Shiite activists accused of plotting against the Sunni monarchy that has ruled Bahrain for more than 200 years.
At least 30 people have died since Feb. 15, when anti-government protests erupted in Bahrain. Four opposition supporters have also died in police custody.
Bahrain is the home of the U.S. Navy's 5th Fleet.
Bahrain last issued a death sentence in 2007, and prior to that had condemned only one person die over the preceding three decades.
That verdict came in the mid-1990s, during the greatest unrest Bahrain had seen before this year's protests.
The Associated Press and Reuters contributed to this report.
