BANGKOK (REUTERS) - An exiled Cambodian opposition leader was denied entry into Thailand on the weekend, ahead of her planned return to Cambodia early next month, a rights group told Reuters on Wednesday (Oct 23).

Ms Mu Sochua, vice-president of the banned opposition Cambodia National Rescue Party (CNRP), arrived at Thailand's Suvarnabhumi Airport on Sunday afternoon, Human Rights Watch told Reuters.

She "was stopped at Thai Immigration for several hours, and finally denied access to Thailand", the rights group said, adding that she was allowed to book a flight to return to the country she had come from.

It was unclear why Ms Mu Sochua was denied entry. Thai police have not responded to Reuters' inquiry.

Reuters has also contacted Ms Mu Sochua and the Cambodian government for comment.

The incident came ahead of a planned return of exiled opposition leaders, including Sam Rainsy and Ms Mu Sochua, to Cambodia on Nov 9, the country's Independence Day, to rally support against Prime Minister Hun Sen's regime.

CNRP founder Sam Rainsy fled to France four years ago, following a conviction for criminal defamation in which he was ordered to pay US$1 million (S$1.36 million) in compensation. He also faces a five-year prison sentence in a separate case.

Last month, Cambodia arrested six political activists for alleged plots to rally in support of Rainsy if he returns, bringing the number of activists for the banned opposition party who have been detained this year to at least 30.

Critics have described Cambodia as essentially a one-party state since the Supreme Court dissolved the CNRP in late 2017, months before elections last year in which longtime Prime Minister Hun Sen's party won all of the seats in Parliament.

The CNRP leader, Kem Sokha, has been detained for two years, the past year under house arrest, while awaiting trial on charges of plotting to overthrow the government.