Cambodians are voting in elections expected to extend the rule of prime minister Hun Sen who's been in power for the past 28 years.
His Cambodia People's Party (CPP) faces an opposition led by Sam Rainsy - back from self-imposed exile after a royal pardon in July.
Mr Rainsy himself is not eligible to stand, but his recent return has strengthened the opposition's campaign.
The BBC reports more than nine million people are expected to cast their votes.
Hun Sen, 60, was among the early voters, casting his ballot shortly after polls opened near his home in Takmau, south of the capital Phnom Penh.
He is one of Asia's longest-serving leaders, and has expressed the desire to stay in power for another decade.
Mr Rainsy, 64, visited a polling station near his party's office in Phnom Penh where supporters greeted him enthusiastically.
The CPP, which enjoys considerable support in the countryside, currently has 90 of the 123 seats in parliament and has been increasing its majority over the years.