The Sunni-led state saw the worst sectarian clashes since the 1990s last month after mainly Shi'ite protestors, emboldened by uprisings in Egypt and Tunisia, took to the streets.
Manama imposed martial law and invited in troops from Saudi Arabia and other Sunni allies to keep order.
Security forces arrested lawyer Mohammed al-Tajer on Saturday, the Bahrain Youth Society for Human Rights and Wefaq, the biggest opposition party, said.
"Security forces stormed his home... (at) 2 a.m.," the rights group said in a statement.
Tajer represented Hassan Mushaimaa, leader of the Bahraini opposition group Haq, who returned from exile in London in February and was arrested last month.
At least two doctors were also detained, said Wefaq politician Mattar Ibrahim Mattar.
Opposition activists have accused the government of trying to intimidate medical staff to discourage them from treating protestors.
"Two doctors were detained," he said. "They arrested Mohammed al-Tajer to put fear also into lawyers."
Wefaq mobilised more than 100,000 protestors during peaceful marches when the government still allowed gatherings.
The party called for a constitutional monarchy but did not join smaller groups in demanding the overthrow of the al-Khalifa ruling family.
Wefaq won nearly half of seats in parliament in last year's elections but complained electoral districts had been re-drawn to prevent Shi'ite candidates from taking a majority.
Wefaq resigned its seats in parliament in protest at the government crackdown. By-elections are expected in May to replace them.
Parliament has little power and the cabinet, appointed by the king, has been headed by the same member of the ruling family for four decades.
The government has good relations with the United States and Bahrain hosts the U.S. Fifth Fleet.