Up to 7,000 people gathered at the National Mosque demanding the resignation of the Prime Minister, Mahathir Mohamad, and the release from prison of Mr Anwar.

The crowd shouted "Long live Anwar" and slogans denouncing Dr Mahathir before being charged by riot police firing tear gas and chemical-laced water cannon.

Mahathir Mohamad: Sacked Mr Anwar a year ago

Protesters replied by throwing bricks, sticks and even flower-pots at the riot police and their trucks. Several were arrested.

After a tense stand-off lasting two hours the crowd dispersed.

Late on Sunday, several hundred Anwar supporters had regrouped and were shouting anti-government slogans in a mostly Malay neighbourhood of the capital. Police presence was light.

One of their leaders said they planned to regroup in Independence Sqare in the heart of the capital on Monday evening to mark the anniversary of Mr Anwar's arrest.

Royal petition

The thousands of protesters - men, women and children - who had gathered for prayers at the mosque, heard Mr Anwar's eldest daughter, Nurul Izzah, read out a petition intended for the Malaysian king.

The BBC's Francis Harrison in Kuala Lumpur: "Mr Anwar says it is a matter of life and death"

In it she called for a royal commission of inquiry into allegations that Mr Anwar was poisoned with arsenic while in jail. He is currrently in hospital.

About 1,000 protesters, including Nurul Izzah, managed to march to the palace to deliver the petition, but most were cornered by riot police.

The protest comes on the eve of the anniversary of Mr Anwar's arrest last September and his assault in police custody the same night.

Anwar Ibrahim: Tests said to show arsenic in his blood

Feelings are running high because Mr Anwar's supporters believe his alleged poisoning was another attempt on his life, and they are outraged that after 10 days of medical tests, the authorities have still not revealed their findings.

Dr Mahathir on Sunday dismissed Mr Anwar's complaint he was poisoned as an attempt to stir up his supporters and discredit the government.

Mr Anwar is currently serving six years after being convicted of corruption. He is also being tried on sodomy charges.

He and his supporters deny any wrong-doing, and say he is the victim of a plot by Dr Mahathir.