By Liz Lee, Lucien Libert and Prak Chan Thul
KUALA LUMPUR/PHNOM PENH (Reuters) - Cambodian opposition veteran Sam Rainsy flew into Malaysia on Saturday and told supporters to "keep up the hope" after promising to return home from self-imposed exile to rally opponents of authoritarian ruler Hun Sen.
Cambodia's government said there was no ban on Rainsy returning, but it described him as "convict Sam Rainsy" and said it would take action against anyone posing a threat to state security - an accusation it has already laid against him.
For decades, one-time finance minister Rainsy, 70, has been an opponent of Hun Sen, 67, a former Khmer Rouge commander who has ruled his Southeast Asian country of 16 million people with an iron hand for 34 years.
"Keep up the hope. We're on the right track," Rainsy said in a message to supporters as he arrived at Kuala Lumpur International Airport. "Democracy will prevail. Democracy has prevailed in Malaysia. Democracy will prevail in Cambodia."
Rainsy fled to Paris in 2015 after a conviction for criminal defamation and faces a five-year sentence in a separate case - charges he says were politically motivated.
Rainsy had originally said he planned to cross to Cambodia from Thailand on Saturday with other leaders of the banned Cambodia National Rescue Party (CNRP), but he was refused permission to board a Thai Airways plane from Paris on Thursday.
After arriving in Kuala Lumpur from the French capital, he did not say whether he still planned to go home. Malaysia has no border with Cambodia.
"The return to the country of the convict Sam Rainsy and his faction as Cambodian citizens is unimpeded," Deputy Prime Minister Sar Kheng said on Facebook. "But the Royal Government of Cambodia has also announced the right to take legal action against any actions that attempt to undermine peace, social stability and security of the state."
In the weeks since Rainsy announced his plan to return, some 50 opposition activists have been arrested in Cambodia.
Police with assault rifles massed at the Poipet border crossing with Thailand on Saturday. In the capital, Phnom Penh, security forces patrolled in pickup trucks during celebrations for the 66th anniversary of independence from France.
An opposition official in Thailand said plans to return on Saturday were abandoned.
"We can say that we achieved at least 70% of our goal," a CNRP official in Bangkok, Saory Pon, told Reuters. "You can see the intimidation, the harassment, the crackdown, the arrests."
On his Facebook page, Hun Sen said he hoped people would enjoy the boat races during an annual water festival on Sunday and Monday. He made no reference to Rainsy.
Rainsy said he would be in Malaysia for "a few days" meeting "like-minded friends" and would speak to parliamentarians on Tuesday.
"Of course I want to go back to my home country," he said.
His freedom to hold meetings appeared to mark a rapid shift in stance from Malaysia, which earlier this week detained and later released the Cambodian opposition party's vice president, Mu Sochua, and two other officials.
Human rights groups have accused countries in the Association of Southeast Asian Nations (ASEAN) of trying to protect Hun Sen by obstructing and detaining his opponents.
"Malaysia deserves kudos," said Phil Robertson of U.S.-based Human Rights Watch. "More countries in ASEAN need to emulate Malaysia going forward if the bloc is ever going to shake the moniker of being primarily a dictator's club."
Rainsy, a founder of the CNRP who usually wears large, rimmed spectacles, has been an opponent of Hun Sen since the 1990s, when Cambodia held its first elections after the devastating era of the Khmer Rouge genocide.
Rainsy also vowed to return home in 2015 despite threats to arrest him, but did not.
The CNRP's leader, Kem Sokha, is under house arrest in Cambodia after being arrested more than two years ago and charged with treason ahead of a 2018 election that was condemned by Western countries as a farce.
(Additional reporting by Richard Lough in Paris, Liz Lee, Ebrahim Harris and Rozanna Latiff in Kuala Lumpur, Patpicha Tanakasempipat and Juarawee Kittisilpa in Bangkok; Writing by Matthew Tostevin; Editing by Helen Popper)