The vice-president of the Bahrain Centre for Human Rights (BCHR) has been detained, the organisation has said.
Sayed Yousif al-Muhafdha was arrested at a protest on Monday in the capital, Manama, which commemorated the killing of two Bahrainis in 1994, it added.
Prosecutors ordered that Mr Muhafdha be held for a week pending an investigation into an accusation he broadcast false information on Twitter.
Last week, a court rejected an appeal by the BCHR's president, Nabeel Rajab.
It upheld his convictions for encouraging illegal gatherings, but cleared him of insulting police and reduced his prison sentence from three years.
Human Rights Watch called the decision "bizarre", and said Mr Rajab was being punished for exercising his right to freedom of association.'Excessive force'
Mr Muhafdha, who has campaigned tireless for the release of Mr Rajab, has been detained on several occasions by the Bahraini authorities.
He was detained at a rally called by the 14 February Coalition for "Bahrain Martyrs' Day", which marks the killing of two protesters in 1994.
Security personnel fired tear gas and stun grenades to disperse them.Security forces attempted to prevent opposition supporters attending the demonstration by closing most roads leading to Manama, but around 100 still managed to gather in the narrow streets of the city's market district.
A BCHR statement said protesters were "attacked... with excessive force", and that 25 men and three women were arrested, among them Mr Muhafdha, who had been there to "monitor and document the situation".
"The BCHR believes that the arrest of Muhafdha is part of an ongoing systematic targeting, harassment and detention of human rights defenders in Bahrain, and in particular those associated with the BCHR," it added.
The organisation also strongly condemned the "continuous crackdown on civilians who choose to exercise their right to freedom of expression and peaceful assembly".
Last month, the government banned all public gatherings and rallies.
Interior Minister Sheikh Rashid Al Khalifah said "repeated abuse" of the rights to freedom of speech and expression could no longer be accepted.