Husam al-Haddad was beaten by security forces late on Friday and died from his injuries, the Bahrain Centre for Human Rights said.
The interior ministry confirmed that al-Haddad had been killed but said he had been attacking police with molotov cocktails.
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At least 50 people have died since February 2011 in protests in the island kingdom, where the Shi'ite majority is fighting for greater political rights from the Sunni monarchy.
The killing of al-Haddad came only one day after prominent activist Nabeel Rajab was sentenced to three years in prison for instigating and participating in anti-government rallies.
