KUALA LUMPUR, Sept 21 (Reuters) - Malaysian police fired water cannon and tear gas on Monday at several thousand demonstrators outside a courthouse where they had gathered in support of detained former finance minister Anwar Ibrahim.

Riot police brandishing shields charged the crowd, pushing protesters away from the courthouse in the second clash between authorities and demonstrators opposed to Prime Minister Mahathir Mohamad in two days.

Some of the estimated 7,000 protesters fled and others took shelter when police turned the water cannon on them, a Reuters correspondent at the magistrates court said.

The protesters, who had gathered expecting Anwar to be arraigned, shouted ``Reformasi'' (Reform) as police sprayed them with water.

``They fired the water cannon and then three shots of tear gas,'' one protester said.

At least 20 demonstrators, including one who tried to chain himself to the courthouse railing, were detained by plainclothes detectives, witnesses said.

More than one hour after riot police fired the water cannon and tear gas, demonstrators continued to regroup outside the courthouse, witnesses said.

Anwar, who was sacked from the cabinet on September 2, was arrested at his suburban home on Sunday after leading 30,000 supporters through the streets of the capital shouting for Mahathir to step down after 17 years in power.

Riot police also fired tear gas and water cannon on Sunday night when about 5,000 protesters converged on the prime minister's official residence in an unprecedented act of defiance.

At least 11 others were detained after Sunday's rare demonstration of public disgruntlement, the Star newspaper said.

Kuala Lumpur's police chief had said Anwar would be indicted for breach of peace, illegal assembly and vandalism on Monday.

Anwar's lawyer had said the former finance minister might be charged with gross indecency. On Saturday, Anwar's adopted brother and a former speechwriter pleaded guilty to being sodomised by him. Anwar denied the allegations, calling them a political frame-up.

But the federal police chief was quoted on Monday as saying Anwar would be held under the Internal Security Act, a draconian national law which provides for indefinite detention without charges, and not produced before the court on Monday.

Inspector-General of Police Abdul Rahim Noor said investigations into allegations of criminal misconduct by Anwar would continue and he could not rule out the possibility that the former cabinet minister would be charged in court at a later date.

Anwar's supporters began gathering outside the magistrates court early on Monday in expectation of his arraignment.

The crowd shouted ``Long Live Anwar,'' ``Mahathir Resign'' and ``Allahu Akbar'' (God is Greatest) while a police helicopter hovered overhead, Reuters correspondents said.

Late in the morning, riot police fired water cannon and tear gas and, walking in a phalanx, pushed demonstrators away from the courthouse with their shields.

``This is not justice. We do not harm people. Is this what you call freedom?'' a 26-year old clerk told Reuters after he was hit with water.

Mahathir on Sunday brushed asided Anwar's incipient reform movement, calling it a ``minor'' distraction, and on Monday he smiled as he passed before a line of dignitaries during a ceremony welcoming Britain's visiting Queen Elizabeth.

The queen was to close the 16th Commonwealth Games on Monday evening in the presence of Mahathir.

Before dawn on Monday, police entered Anwar's suburban home and seized religious documents and video cassettes while more than 100 riot police stood guard outside.

Police posted extra guards around the mansion where Queen Elizabeth was staying. She was due to meet Mahathir at 11 a.m. (0300 GMT) as scheduled, British officials said, adding that there had been no changes in her four-day schedule.

Anwar's arrest came two and a half weeks after Mahathir dismissed him from the cabinet and on the eve of the final day of the 16th Commonwealth Games.