Malaysian immigration authorities detained Sochua after Cambodia sought her arrest on the grounds that she and other self-exiled opposition leaders were plotting a coup in trying to return home.

"Our principle, in ASEAN in particular, generally is that we don't interfere in the internal affairs of other countries," Mahathir said referring to the Association of Southeast Asian Nations.

"We do not want to let them use Malaysia as a base for struggle in other countries. We wanted to deport her ... now we are trying to find any country that can take her."

Sochua's detention came as Sam Rainsy, the founder of the opposition party, prepared to fly from Paris to Bangkok, vowing to return to Cambodia on Saturday to lead demonstrations against the one-party rule of longtime Prime Minister Hun Sen.

Sochua, 65, flew into Kuala Lumpur on her US passport and was being questioned at the airport on Thursday.

It was unclear if any of the dozens of opposition figures abroad would reach Cambodia for the planned rallies. Thailand said it will not allow transit for Sam Rainsy, and Malaysia this week detained two other opposition leaders trying to fly to Thailand.

A former women's minister, Sochua is vice president of the banned Cambodia National Rescue Party (CNRP) and one of the highest-profile opposition leaders detained in Hun Sen's efforts to prevent their return.

Thailand denied her entry on October 20.

On Wednesday, she held a news conference in Indonesia to denounce a crackdown by Cambodian authorities, who have arrested at least 48 opposition activists this year and deployed troops along the borders in recent weeks.

"What do they fear?" she asked. "They totally fear the determination of the people of Cambodia, who are responding positively to our appeal, which is that together we will build a better Cambodia."

She later flew to Malaysia, only to be detained.

Mu Sochua fled Cambodia in 2017 amid a mass crackdown on the opposition. The party's leader, Kem Sokha, was arrested on treason charges and remains under house arrest.

Cambodia's Supreme Court later dissolved the CNRP. The party of Hun Sen, who has ruled Cambodia for more than three decades, went on to win all the seats in parliament in a general election last year.

Cambodian national police spokesman Chhay Kim Khoeun said Mu Sochua had been charged in connection with a plot to topple the government.

"There is an arrest warrant for her. We really want her, but we don't know what is Malaysia's position," he said.

Australian Associated Press