Cambodia's self-exiled opposition leader has urged Japan, his nation's biggest donor, to use its leverage and convince the autocratic government to make sure that the July election is democratic.

Sam Rainsy, whose Cambodia National Rescue Party has been banned and opposition activists harassed, says he's hopeful that Japan and its business community can send a message to Prime Minister Hun Sen that he needs to restore democracy or face isolation.

Rainsy is in Tokyo to meet with Japanese business and political leaders, seeking support for his comeback amid growing democratic setbacks ahead of the July polls.

Rainsy has urged boycotting the vote under current conditions.

Hun Sen has held power for more than three decades and declared his intention to remain for at least two more five-year terms.