Opposition leader Sam Rainsy has returned to Cambodia, days before a general election.
Mr Rainsy had been living in France after being jailed in absentia in 2010 on charges he said were politically motivated.
But he was granted a royal pardon last week and was greeted by thousands of cheering supporters as he arrived back in Phnom Penh.
Cambodia goes to the polls on 28 July, with PM Hun Sen expected to win.
Hun Sen, one of the world's longest-serving prime ministers, has been in power in various coalitions since 1985.
Cambodia had come under pressure from the US over the issue of Mr Rainsy, with a State Department spokeswoman calling on the authorities to allow him to return "without fear for his personal safety or incarceration".
Last week a number of US lawmakers also called for aid to Cambodia to be cut unless the polls were free and fair.
The opposition leader was granted a royal pardon on 12 July at the request of Hun Sen, Cambodian officials said.
'Rescue the country'
Sam Rainsy, 64, arrived back in Cambodia on a flight from Bangkok.
"I have come home to rescue the country,'' he told a crowd of supporters at the airport. "I am happy to be here!''
His Cambodia National Rescue Party will face Hun Sen's Cambodian People's Party - which currently has 90 of the 123 National Assembly seats - in the polls.
The last election, in 2008, was criticised by European Union monitors, who said it fell short of international standards.
The governing party dominated the media and the National Election Committee, and tens of thousands of people were disenfranchised, the monitors said.
Sam Rainsy is currently unable to run in these polls because his name has been removed from the electoral register, but an opposition spokesman said his presence was expected to "galvanise activists and voters".
The spokesman also said the party was looking into ways to register Mr Rainsy as a candidate.
He had been living overseas since 2009, after he was stripped of immunity from prosecution and faced charges.
In 2010, he was sentenced in absentia to 10 years in jail after being found guilty of manipulating a map to suggest Cambodia was losing land to Vietnam.