Sam Rainsy, Cambodia's opposition leader, returned from a four-year exile on Friday after being granted a royal pardon.
He immediately joined the campaign to unseat long-serving Prime Minister Hun Sen in a general election on July 28, Reuters reported.
Later that day he addressed a huge crowd at Freedom Park in the Cambodian capital, thanking King Norodom Sihamoni for the pardon over charges he has described as politically motivated.
"I have received my full freedom and will use this freedom to protect Cambodians in the whole country."
The 64-year-old, French-educated former finance minister had been sentenced in absentia in 2010 to 12 years in prison on charges of falsifying maps to contest a new border agreed by Cambodia and Vietnam, Reuters wrote.
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US-based Human Rights Watch said at the time that the charges showed Hun Sen was "no longer interested in even the pretence of democracy" in Cambodia.
However, it was Hun Sen — who has ruled the country since 1985 — who asked the king to pardon Rainsy.
According to Reuters, the request may have been in response to US and European Union criticism, as well as allegations of electoral misconduct.
The prime minister's Cambodian People's Party has 90 of the 123 seats in the National Assembly, and Hun Sen recently said that he intended to stay in office until he was 74, the Associated Press wrote.
He may be helped in his bid by the recent 7.2 percent growth in Cambodia's economy, which the Asian Development Bank attributed to higher consumption and investment.
Opponents of Hun Sen's regime have said that he manipulates government and the judiciary to stay in power.
He oversaw the removal of 28 opposition lawmakers last month linked to Rainsy's party.