A Bahraini man was found dead on Saturday after clashes with riot police in the village of Shakhoura, the opposition party Wefaq said, a day before Bahrain's Formula One Grand Prix race.
Violence has escalated in the run-up to the Grand Prix, a prestigious event cancelled in Bahrain last year due to political turmoil. State officials count on the running of this year's race to signal to the world a return to normalcy in the Gulf Arab state, a stance rejected by opposition protesters.
Wefaq named the dead demonstrator as Salah Abbas Habib, 37, and said his body was found on the roof of a building. Wefaq said Habib was part of a group who were beaten by police during clashes late on Friday night.
Wefaq, the leading party among Bahrain's restive Shi'ite Muslim majority population, published a photograph taken of Habib's body splayed on a corrugated iron rooftop.
Bahrain's Interior Ministry said via Twitter that it was launching an investigation.
Bahrain's Sunni Muslim ruling family has been grappling with opposition unrest since an uprising for democratic change was launched last year, inspired by grassroots revolts in Egypt and Tunisia.
The government initially crushed the protests by introducing martial law and bringing in Saudi troops. But opposition parties later resumed organising street rallies and youths in mainly Shi'ite villages have clashed with riot police daily.
Thirty-five people, including some security personnel, had been killed by the time martial law was lifted in June but the toll has risen to at least 81 since then, including Habib, according to Wefaq.
Wefaq official Sayed Hadi al-Mousawi said it was not clear what caused Habib's death. "We haven't got the body because the official investigators have surrounded the area but we understand he was beaten severely. His colleagues with him last night were beaten with batons and the butts of rifles used to shoot tear gas and bird shot."
Activists said that more than 100 protest organisers were arrested this week. Police acknowledged arresting some people.
Wefaq and other opposition parties want a shift to parliamentary democracy, reducing the powers of the ruling Al Khalifa family, although some protesters reject the monarchy altogether.
The government accuses the protesters of having a Shi'ite sectarian agenda back by Iran, something they deny.