His return came less than a week ahead of the anniversary of last year's contentious July 28 election, which his Cambodia National Rescue Party claims was rigged. Its lawmakers have boycotted their seats in the National Assembly and are demanding reforms and new elections.
Tensions over the yearlong political deadlock were ramped up this week when seven lawmakers and another party activist were arrested after violence broke out when party members sought to stage a demonstration at a park authorities had closed to them. The lawmakers were charged with insurrection and other offenses, which could see them jailed for up to 30 years.
Sam Rainsy said on his arrival that he would head to the area of Tuesday's confrontation, Freedom Park, which he wants reopened as soon as possible so that people could exercise freedom of expression.
But in his remarks made over a loudspeaker to a crowd at the airport, he also said he expected a political resolution to the country's political crisis, and declared that the detained opposition party politicians would be released.
Both the opposition party and Prime Minister Hun Sen said in April that they were close to settling the deadlock, but no deal was made at the time. While there has been no public sign of detente between the two sides, Hun Sen announced later that he would accede to one of the opposition demands, and allow them to operate a television station.
Hun Sen has been in power for almost three decades, and while Cambodia is formally democratic, his government is authoritarian and known for intimidating opponents. Sam Rainsy is his most enduring opponent, and his party's strong showing in last year's polls — even according to the disputed official results — poses the strongest challenge to Hun Sen in many year.
The latest arrests drew criticism from human rights groups and Western governments. The violence was unleashed when local security guards used by authorities to break up protests began attacking the demonstrators.