KUALA LUMPUR, Malaysia — Police arrested Malaysia’s leading opposition figure yesterday on suspicion that he sodomized a male aide — the second time in a decade he has faced the accusation that could send him to jail for 20 years and hobble his campaign to bring down the country’s government.

Anwar Ibrahim, 61, dismissed the accusation — made last month by the 23-year-old aide — as a “malicious” fabrication by the same ruling establishment that jailed him for sodomy and corruption in 1998.

“This is not a criminal case but a political case,” the vice president of Mr. Anwar’s People’s Justice Party, Azmin Ali, said.

The arrest will give some breathing space to Prime Minister Abdullah’s government, which is struggling to cope with internal dissent after recent election losses, and threats by Mr. Anwar to seize power by mid-September. Mr. Abdullah’s administration has denied any role in the sodomy allegation against Mr. Anwar, the only Malaysian politician to face the accusation.

His lawyers said he will remain in detention at least overnight and possibly longer. “The excuse is that they need to take further statements” from Mr. Anwar, one of his lawyers, Sankara Nair, said.

Mr. Anwar had agreed to submit himself voluntarily for questioning at police headquarters yesterday, but was arrested outside his house an hour before the designated time by a posse of policemen, some wearing ski masks.

For the past year, Mr. Anwar has worked tirelessly to bring the ideologically divided opposition into an alliance that recently shook up the National Front coalition that has ruled the country since it gained independence from Britain in 1957.

In March, the Front lost its two-thirds majority in Parliament as the opposition alliance won 82 of the 222 seats and took control of governments in five of Malaysia’s 13 states. The outcome radically changed the political landscape of a country long accustomed to semi-authoritarian government.

[Some 300 demonstrators demanded Mr. Anwar’s release on Tuesday during a protest in the center of the capital, Bloomberg News reported.

The demonstrators chanted “reformasi,” the Malay word for “reform,” repeating calls made during demonstrations in support of Mr. Anwar in 1998. They shouted that he was innocent and should be released. Others started a candlelight vigil as darkness fell.

Mr. Anwar’s party on Tuesday appealed for calm among its supporters.

“We have been in touch with Anwar and he has called on his supporters and the people of Malaysia to remain calm and reject any attempt at provocation,” a deputy president of the People’s Justice Party, Syed Husin Ali, said.]