Mashaima had only returned to Bahrain on 26 February after terrorism charges against him were dropped as part of an earlier peace offering from the government to the opposition.
Human rights activist and Haq member Abduljalil al-Singace, who was released in February after six months in jail, was also detained, the opposition said. The government has yet to confirm the arrests.
On Wednesday, security forces fired tear gas and shotguns to clear protests at Manama’s Pearl Square, in the worst day of violence since demonstrations began last month.
The opposition said three demonstrators were killed, while the government said two police officers also died.
United States Secretary of State Hilary Clinton said that Bahrain's rulers, and those of neighbouring countries who sent troops to help quash the protests, were "on the wrong track".
Meanwhile, British Prime Minister David Cameron urged Bahrain’s King Hamad to pursue "reform, not repression."