Witnesses said hundreds of anti-riot police on Thursday fired tear gas to disperse more than 1000 protesters on a main highway to Saudi Arabia, demanding the release of the rights activist Abd al-Hadi al-Khawaja.

Some 30 protesters were arrested, including a spokesman for a committee fighting for al-Khawaja's release.

The clashes occurred around midnight as police set up road blocks around the capital Manama to stop cars displaying pictures of al-Khawaja.

The protesters hurled stones and raised anti-government slogans.

Controversial arrest

Khawaja, head of the now-banned Bahrain Centre for Human Rights, was arrested last month for calling for the resignation of Prime Minister Khalifa bin Salman Al Khalifa, whom he blamed for the country's economic woes.

His detention and the consequent closure of his human-rights centre has provoked criticism from international rights groups and the United States.

Bahrain's King Hamad bin Isa Al Khalifa has stood by the prime minister - who is also his uncle - and says he will not tolerate criticism of senior government officials.