Rainsy, who has lived in self-exile in France since 2015 to avoid jail for convictions he says are politically motivated.
Cambodian opposition figurehead Sam Rainsy arrived in Malaysia on Saturday as he sought to return home to rally his supporters against strongman Hun Sen, while security was beefed up at the kingdom's borders and in Phnom Penh.
Rainsy, who has lived in self-exile in France since 2015 to avoid jail for convictions he says are politically motivated, had long promised a dramatic return on November 9, Cambodia's Independence Day.
But Hun Sen, who has ruled Cambodia since 1985, vowed to bar him and other figures from his party, which was outlawed ahead of much-criticised elections last year.
After being blocked from getting on a France-Thailand flight on Thursday, Rainsy boarded a plane a day later and arrived at the main airport serving the Malaysian capital Kuala Lumpur, according to an AFP journalist.