Rows of military personnel, barbed wire and security controls were deployed by the army and police at the border with Thailand, a series of images published on social networks by the Cambodian Centre for Human Rights (CCHR) showed, reports Efe news.
"Security is tightening at the #Cambodia border with Thailand, in light of the announced return of former CNRP (Cambodian National Rescue Party) senior leaders on 9 November 2019," CCHR said in a Twitter post.
Rainsy, the leader of the opposition Cambodian National Rescue Party (CNRP) who on Thursday was prevented from boarding a flight from Paris to Bangkok on the Thai national carrier despite having a French passport, said on Friday night on his Facebook page that he was boarding a plane to another destination, without giving details.
The government of Cambodian Prime Minister Hun Sen, who has ruled since 1985, has described the plan as a coup attempt and has pledged to prevent it by all means.
In addition, Phnom Penh on Friday cancelled passports for 12 senior opposition leaders outside the country who had also announced their intention to return.
Authorities have placed posters at the borders with photographs of the main members to prevent their entry into the country on Saturday.
Rainsy has been living in exile since 2015 to avoid prison sentences, which he says were politically motivated.
In recent days, Malaysia has prevented three Cambod linked to the CNRP from travelling to Cambodia, including Mu Sochua, the party's Vice President, who has a US passport.
Hun Sen, the world's longest-serving Prime Minister, has ordered the arrest of opponents whom he calls "traitors".
He tightened his grip on the country in 2017 after the Supreme Court dissolved the CNRP, considered the only real threat to the ruling Cambodian People's Party (CRP) ahead of the 2018 elections. The CPP subsequently won all available parliamentary seats, effectively making the country a one-party state.
CNRP co-founder Kem Sokha was arrested for treason on September 4, 2017, which - similarly to Rainsy's charges - is also widely seen as being politically motivated. After a year in pre-trial detention, he was released on bail last September and placed under house arrest, where he remains.
( With inputs from IANS )