Malaysian opposition leader Anwar Ibrahim has been arrested in connection with allegations of sodomy, a crime in the Muslim country, made by an aide.
Mr Anwar is due to spend the night in custody in the capital, Kuala Lumpur, following his arrest by armed police near his home earlier, his lawyer said.
The arrest came shortly before Mr Anwar was due to give a statement to police over the allegations, which he denies.
The former deputy PM says they are aimed at ending his political comeback.
The move is likely to exacerbate the political tensions that have emerged since the opposition's unprecedented gains in the general election of March 2008.
Tian Chua, information chief for Mr Anwar's People's Justice Party (Parti Keadilan Rakyat), said his leader appeared to be in good spirits:
"He's quite cheerful. He's fresh, relaxed and hoping this will be over soon."
Mr Anwar has been in a tense stand-off with police since a former male aide accused him of sodomy two weeks ago.
Sodomy, even between consenting adults, is punishable by 20 years' imprisonment in Malaysia.
The 60-year-old opposition leader had been scheduled to report to police on Wednesday to be interviewed over the allegations.
But an hour before the meeting, armed police wearing balaclavas took him into custody outside the gates of his home, his lawyer said.
"The manner in which they surrounded our vehicle, it put a lot of fear and intimidation into us," Sankara Nair said.
Mr Nair added that he did not know how long his client would be held in custody. He quoted police as saying they needed to take further statements.
Deputy National Police chief Ismail Omar said Mr Anwar was being questioned at police headquarters.
"We have to record his statement to complete our investigations," he told the French news agency AFP. "Once the facts are in we can make a decision."
Police can detain the opposition leader for 24 hours, after which they must apply for a court order for further detention.
Mr Anwar's arrest will be seen as provocative by Malaysian opposition groups.
Police used tear gas and water cannon against Mr Anwar's supporters in 1998
When he was arrested on similar charges 10 years ago, his supporters staged large demonstrations.
A BBC correspondent in Kuala Lumpur, Robin Brant, says it is almost certain that the same thing will happen again now.
The allegations come only weeks after Mr Anwar said he was in a position to launch a challenge to the ruling coalition, with the help of government defectors.
The opposition leader made his claim at a time when Prime Minister Abdullah Badawi is already under intense pressure to resign over poor election results and high fuel prices.
Mr Abdullah has said he will leave office in 2010, defying pressure to step down this December.