But Hun Sen, an authoritarian leader in power since 1985, vowed not to allow Rainsy to return, saying he would be arrested on sight, and has sought support from regional neighbours to thwart the opposition's plans.
Sam Rainsy, self-exiled head of the now-banned Cambodia National Rescue Party, says he was not allowed to get on a Thai Airways flight from Paris to Bangkok on Thursday, despite having purchased a ticket.
"They said they have received from very high up the instruction not to allow me to board", he told reporters at Paris-Charles de Gaulle Airport. "I'm very shocked, I'm very disappointed. I want to go back, my people are waiting for me".
Hun Sen, who has ruled Cambodia for more than three decades, has described the planned return of opposition leaders overseas as a bid to stage a coup d'etat. "A violent crackdown or an arrest of Rainsy would nearly certainly seal Cambodia's EBA fate, but the Cambodian government is taking actions to ensure that this won't happen".
Mr Rainsy is one of Cambodia's leading opposition figures but he has been living in Paris since 2015.
Ms Mu Sochua, vice-president of the now-dissolved Cambodia National Rescue Party (CNRP), was detained by immigration officials at Kuala Lumpur International Airport at around midnight on Wednesday, two weeks after she was turned away at Thailand's Suvarnabhumi Airport.
Earlier this week, two other Cambodian opposition activists were arrested in Malaysia as they sought to fly to Thailand.
Jerald Joseph, an official of the Malaysian Human Rights Commission, said the Cambodians would stay in a hotel for now.
In 2017, fellow CNRP leader Kem Sokha was jailed in Phnom Penh on charges branded as political; after one year he was released on bail but has been under house arrest since.
Around 50 opposition supporters have also been arrested in Cambodia in recent months, for allegedly supporting a coup. A lot of them have convictions or charges pending - many of which have been handed down in absentia.
The ASEAN Parliamentarians for Human Rights also condemned her detention and welcomed her release.
Thai Prime Minister Prayut Chan-o-cha, however, ordered that Rainsy not be permitted to enter the country.
While professing a desire to arrest Rainsy and other CNRP leaders, Hun Sen has gone to great lengths to ensure that they can not enter the country.
Hun Sen said Sochua was stopped in Malaysia because authorities were upholding a tenet of the 10-member Association of Southeast Asian Nations not to "interfere in each other's internal affairs". "They generally don't like to push political issues in other states".
He tells NPR that while stopping the opposition leaders from transiting through their borders is, in a way, getting involved, ASEAN's tendency has always been "to act in consensus ways, and that tends to favor established rulers, most who have been authoritarian throughout the time of ASEAN".
Ear Sophal, a Cambodian-American associate professor of World Affairs and Diplomacy at Occidental College, said a third party could have changed Rainsy's ticket after he tweeted it, or the airline may have considered his ticket invalid because he was barred from the country.
"This would be in line with Mr Rainsy's strategy which has been consistent over the past years: to try to merit some form of worldwide intervention", Mr Ou said. "Rainsy enjoys a grand gesture and a grand gesture is sometimes valuable".
Joanne Mariner, Amnesty International's research director for Southeast Asia, questioned the action by countries in the region to block opposition leaders and activists from travelling home.
In mid-August, the European Union announced it had completed its investigation into the country's increasingly authoritarian behavior and would decide on trade sanctions within three months that could impact its $5 billion garment industry.
Eyler also says it's an interesting time to be in Cambodia, as the country is heading into a dry season after an already devastating drought that is expected to decrease the annual fish harvest significantly and potentially throw the country into a food crisis.
"Sam Rainsy is still very popular in Cambodia", Virak Ou, political analyst in Phnom Penh told the BBC, explaining that Cambodian politics is polarised between Hun Sen and Mr Rainsy.