Tunisia's state news agency says a police station was attacked by relatives of a known radical militant after he was arrested, resulting in the wounding of three members of the security force.

Responding to extremist threats ahead of October elections, Tunisian police have been arresting those suspected of ties with radical groups.

Interior Ministry spokesman Mohammed Ali Aroui said police in the southern town of Douz resorted to tear gas to disperse a crowd which had gathered, and arrested 21 members of the extended family late Wednesday.

Press reports Thursday say the police were injured by birdshot fired by some shotgun-wielding protesters.

Militant groups with ties to al-Qaida have vowed to disrupt election set for October that would complete the country's democratic transition.