Rainsy had planned to return to lead the people's "fight for a better life", describing the long-time Prime Minister Hun Sen, who has ruled Cambodia since 1985, as "a brutal dictator", the BBC reported.
"I am extremely shocked because the people need me in Cambodia," Rainsy said at Charles de Gaulle airport after he was turned away at the Thai Airways counter.
He intended to fly to Bangkok and then reach Cambodia, but he claimed that the check-in desk "said they have received from very high up the instruction not to allow me to board".
Rainsy has vowed to get another flight and stick to his plan to make it to Cambodia by Saturday, the country's independence day.
Meanwhile Hun Sen said Rainsy that would be arrested if he tried to enter the country. He also asked neighbouring countries to bar him from transiting, and advised airlines to not accept him as a passenger, the BBC said.
On Thursday, Malaysia briefly detained Rainsy's deputy Mu Sochua, Vice President of his outlawed Cambodian National Rescue Party (CNRP), who hoped to join him in Cambodia.
Rainsy is one of Cambodia's leading opposition figures but he has been living in Paris since 2015. He faces several lawsuits at home which his supporters say are all politically motivated.
His CNRP was for some time seen as the biggest threat to Prime Minister Hun Sen's rule. It was dissolved in 2017 by the Supreme Court, based on a complaint from the government that it was conspiring with the US to overthrow it.
( With inputs from IANS )