The Gulf state saw deadly clashes last month after protesters, emboldened by uprisings in Egypt and Tunisia, took to the streets. Manama imposed martial law and invited in troops from Gulf neighbors 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.

Al-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. "Two doctors were detained," he said. "They arrested Mohammed Al-Tajer to put fear also into lawyers."

Opposition activists have accused the government of trying to intimidate medical staff to discourage them from treating protesters.

Wefaq called for a constitutional monarchy. It won nearly half of seats in Parliament in last year's elections but complained electoral districts had been redrawn to prevent the party's 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.