The leader of the Cambodia National Rescue Party (CNRP), who has been living in France since 2015, said he was "shocked" and "disappointed" by the move, and will "buy another ticket with another company" to return to Cambodia.
Self-exiled Cambodian opposition party founder Sam Rainsy said on Wednesday he would not be deterred by efforts to prevent his return home "to be with my people".
"It is outrageous that Malaysia and other ASEAN countries are doing the dirty bidding of their counterparts fearful of legitimate political opposition", said APHR board member Kasit Piromya.
A member of the ASEAN Parliamentarians for Human Rights, Teddy Baguilat, welcomed their release and urged Thailand to follow Malaysia's example by allowing the Cambodian party's leader, Sam Rainsy, to enter Thailand. Instead, they have to live with constant threats of carnage which have been heightened in recent weeks by government propaganda in the form of documentary that compares the CNRP with Islamic State warning Cambodia could be the world's next Aleppo if Sam Rainsy has his way.
These exiled politicians have previously openly stated their desire to unseat Cambodian Prime Minister Hun Sen, an autocratic leader who has clamped down on opposition.
He said Cambodian Prime Minister Hun Sen "should get the message loud and clear that his persecution of the political opposition will not be exported wherever he wishes it to be".More news: BritBox Launches In The UK After Striking Channel 4 Deal
He also faces a five-year prison sentence in a separate case.
Mahathir's stand mirrored that of Thailand, which this week also barred the Cambodian opposition officials from transiting through the country.
Rainsy said on Thursday the Thai flag carrier "said they had received from very high up the instruction to not allow me to board".
"We want to deport her, but people (ask her) to come back here, now we are trying to find any country that can take her", he told a press conference after the National Government Pledge Committee Meeting earlier today.
"We are a pro-democracy government".
Cambodian officials have repeatedly warned that if the opposition leaders did make it back, they would immediately be arrested.More news: Super Smash Bros Ultimate Terry Bogard DLC detailed and released
"According to our commitment to ASEAN, we will not interfere in each other's internal affairs, and we will not allow an anti-government person to use Thailand for activism", the former junta-leader turned elected premier told reporters in Bangkok. Most if not all have convictions or charges pending against them, including inciting armed rebellion despite their avowedly nonviolent intentions.
In response to Mr Prayut's denial of access to Thailand, Mr Rainsy told Radio Free Asia on Wednesday: "We have already committed that we will arrive in Cambodia to rescue Cambodian people".
"They want freedom, they want justice for the Cambodian people".
Cambodia has sent troops to its border and called on neighbouring countries to arrest exiled dissidents after accusing them of plotting a coup as the European Union reviews trade privileges that are essential to the economy.
"Toppling the illegal Hun Sen regime is a legal and legitimate act because Hun Sen is an usurper who has stolen power from the people through a fake election in 2018", the appeal stated.More news: Boeing submits Human Lander System for NASA’s Artemis moon missions