* Rainsy says democracy will prevail
* Cambodian security forces on high alert
* Opposition official says nobody will return on Saturday (Updates with Sam Rainsy entering Malaysia)
By Liz Lee, Lucien Libert and Prak Chan Thul
KUALA LUMPUR/PHNOM PENH, Nov 9 (Reuters) - Self-exiled Cambodian opposition figure Sam Rainsy landed in the Malaysian capital of Kuala Lumpur on Saturday after promising to return home to rally opponents of authoritarian ruler Hun Sen.
"Keep up the hope. We are on the right track," Rainsy said on arrival at Kuala Lumpur International Airport in a message to supporters. "Democracy will prevail. Democracy has prevailed in Malaysia. Democracy will prevail in Cambodia."
Asked whether he planned to return to Cambodia he said: "I cannot say anything. I do not deny, I do not confirm."
The veteran opposition figure had planned to return to Cambodia on Saturday, Independence Day, in what Prime Minister Hun Sen characterised as an attempted coup against his rule of more than three decades.
But Sam Rainsy was blocked from boarding a Thai Airways flight to Bangkok in Paris on Thursday. He and other leaders of his banned opposition party have said they want to return to Cambodia by crossing the land border with Thailand.
Malaysia has no border with Cambodia.
An official of Rainsy's banned Cambodia National Rescue Party (CNRP) in Thailand said that nobody would be returning on Saturday.
"We will be returning as soon as possible," Saory Pon, General Secretary of the Cambodia National Rescue Party Overseas told Reuters, complaining that some party officials in Thailand had been harassed and followed by security services.
Cambodian government spokesman Phay Siphan said that if Sam Rainsy did return he would face outstanding charges against him in court.
"If he comes to cause instability and chaos, we will destroy him," he said.
Some 50 opposition activists have been arrested in recent weeks.
In Cambodia's capital, Phnom Penh, security forces patrolled in pickup trucks on Saturday, which marks Cambodia's 66th anniversary of independence from France. On Sunday and Monday, Cambodia celebrates an annual water festival.
Police armed with assault rifles lined up at Cambodia's Poipet border crossing with Thailand, where Sam Rainsy had said he planned to cross, pictures posted on Twitter by the independent Cambodian Center for Human Rights showed.
Rainsy, a founder of the CNRP, fled four years ago following a conviction for criminal defamation. He also faces a five-year sentence in a separate case. He says the charges were politically motivated.
The 70-year-old former finance minister, who usually sports large, rimmed spectacles, has been an opponent of Hun Sen since the 1990s. He also vowed to return home in 2015 in spite of 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.
Before Sam Rainsy's failed attempt to fly to Thailand, Malaysia detained Mu Sochua, his party's U.S.-based vice president, at an airport before releasing her 24 hours later along with two other Cambodian opposition leaders detained earlier.
"We will continue our journey home," Mu Sochua said on Twitter on Saturday morning. "9 November is marked in history as our struggle for democracy."
Rights groups have accused Malaysia, Vietnam, Cambodia and Thailand of detaining and returning critics of neighbouring governments, even those with United Nations refugee status. (Additional reporting by Richard Lough in Paris; Liz Lee, Ebrahim Harris and Rozanna Latiff in Kuala Lumpur, Patpicha Tanakasempipat in Bangkok; Writing by Matthew Tostevin; Editing by Robert Birsel and Christian Schmollinger)
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