Condemning state "repression of demonstrations calling for political reforms," the Bahrain's main Shiite opposition group Wefaq said Saida Fadhel Mirza al-Obeidi, 22, was struck in the head on March 1 in Diraz, a village east of Manama.
Doctors said his skull was fractured.
However, Wefaq hailed "the peaceful and disciplined character" of a protest which flooded the streets of the capital on Friday, saying hundreds of thousands had taken part in "the largest rally in the history of Bahrain".
Policemen fired tear gas at a group of protesters but the rally was largely peaceful and no one arrested, said Nabil Rajab, an activist who heads the Bahraini Centre for Human Rights.
The royal palace, in a statement, said the demonstration was able to go ahead on the instructions of King Hamad who had called for citizens to be allowed to express themselves freely in a peaceful rally.
Protesters called for political reforms in Bahrain, which is ruled by the minority Sunni Al-Khalifa family, and for an elected government, Rajab said.
The Shiite-led opposition demands constitutional changes that would reduce the power of the dynasty.
Tensions have remained high in Bahrain since a deadly crackdown last year after a month of Manama street protests.
According to an independent probe, 35 people were killed in last year's unrest, including five security personnel and five detainees tortured to death while in custody.