The activists assaulted a group of several dozen demonstrators who were demanding the resignation of Osh Governor Jantoro Satybaldiev and other officials.
The rally had been organized by a nongovernmental organization called Protection of Legality.
"The assailants beat up people who had gathered [in front of the regional administration's building]," Protection of Legality Chairman Abdykaim Kangeldiev told RFE/RL. "Among them were strange young men. One of them hit me in the stomach. Another one hit me in the eye. Others shouted at me: 'Go away!' I told them I wouldn't leave and that they will be brought to justice. As it appeared, those hooligans had come out of the office of [Osh First Deputy Governor] Kushbak [Tezekbaev]."
Tezekbaev denied any involvement in the incident.
The attackers told journalists they were members of a group calling itself the Committee to Protect the People's Revolution, in reference to last year's ousting of President Askar Akaev.
(with material from AKIpress)