Riot police repeatedly fired water cannon and tear gas at several thousand demonstrators who converged on a courthouse in the heart of the capital where they had expected sacked finance Minister Anwar Ibrahim to be arraigned.

It took police brandishing shields more than three hours to disperse the protesters and seal off a large swathe of central Kuala Lumpur, including Merdeka (Freedom) Square where 30,000 demonstrators had congregated on Sunday.

Virtually all the businesses and shops around the courthouse closed after the clashes erupted.

Plainclothes detectives arrested at least 50 people, including several people who said they had come down to the street during their lunch break, witnesses said.

The angry crowd had assembled in a show of support for Anwar, who was sackedby prime minister Mahathir Mohamad on September 2 and arrested late on Sunday night when riot police broke into his suburban home.

The crowd intermittently shouted ``Long Live Anwar,'' ``Mahathir resign,'' ``Allahu Akbar'' (God is Greatest) and ``Reformasi'' (Reform).

Demonstrators distributed leaflets which said: ``We demand that Dato Seri Mahathir step down.''

The protest took place on the final day of the 16th Commonwealth Games, which were intended as a showcase for Malaysia's economic development but have been overshadowed in recent days by domestic political upheaval.

Anti-government protesters even took their anger out on Britain's visiting Queen Elizabeth, who was to close the Games later on Monday. Some of them gave her a thumbs down gesture after she gave them an unwitting cheery wave as she drove by.

Australian athletes, the biggest contingent and biggest winner of the first games in Asia, were ordered to stay away from Merdeka Square.

Anwar, 51, was arrested after leading some 30,000angry demonstrators in a rare display of public outrage through the heart of Kuala Lumpur, shouting for Mahathir, 72, to step down after 17 years in power.

About 5,000 of those demonstrators broke away on Sunday and converged on Mahathir's official residence, where riot police dispersed them with bursts of water cannon and tear gas.

The federal police confirmed on Monday that Anwar, under investigation for sodomy, treason, corruption and other crimes which he denies, was being held under the Internal Security Act, a sweeping law that provides for detention without trial.

Opposition leader Lim Kit Siang said news that Anwar was being held under the ISA indicated the government had been unable to confirm the criminal allegations against Anwar, which have been widely publicised in the press.

``There is concern as to whether this will be a prelude to a larger crackdown against civil liberties,'' Lim said, citing the arrest of more than 100 opposition leaders and activists in 1987 underMahathir.

``Mahathir is becoming a virtual dictator who is in the process of killing democracy and destroying human rights in this country,'' said president Syed Husin Ali of the opposition Parti Rakyat Malaysia.

Police said the youth leader of Mahathir's United Malays National Organisation (UMNO) party, Ahmad Zahid Hamidi, was arrested on Monday under the ISA.

Zahid was close to Anwar and had spoken out against cronyism and corruption at the party's annual general assembly in June, provoking anger among Mahathir's supporters who saw the campaign as a threat to the prime minister.