Millions of Thais are voting in a general election boycotted by the opposition and blighted by protests.
Anti-government protesters say they will disrupt voting across Thailand and continue their campaign to force Prime Minister Yingluck Shinawatra to quit.
Yingluck voted soon after polls opened on Sunday near her Bangkok home.
The prime minister, who won the last election in 2011, called the vote to head off mass protests that began in November.
Her opponents took to the streets after her government tried to pass an amnesty law that would potentially have allowed her brother, Thaksin, to return from exile.
Thaksin, a former prime minister who fled during a court case in 2008, is reviled by the protesters, who say he controls the government from abroad.
Security is heavy throughout Thailand, with vast areas under a state of emergency because of the protests.
Security officials said about 130,000 personnel would be deployed across Thailand on Sunday, including 12,000 in Bangkok.
Polling stations open at 08:00 (01:00 GMT), but there has been little campaigning and it is unclear how many Thais will turn out.
Polling is likely to be severely disrupted in Bangkok and the south, strongholds of the opposition Democrat Party, which is boycotting the election.
Trouble broke out in Bangkok on Saturday when pro-government groups tried to access a building that was storing ballot papers.
Anti-government protesters had blockaded the building, and opened fire with handguns and rifles, sending journalists and passers-by fleeing for cover during a 30-minute gun battle.
Footage from the scene showed pro-government protesters nursing injuries, and anti-government protesters firing guns.
US photojournalist James Nachtwey was hit in the leg by a bullet, but was not seriously injured.
“I consider myself extremely lucky,” Nachtwey told the Wall Street Journal.
Correspondents say less unrest is expected in the rural north and east, because Yingluck’s Pheu Thai party has such overwhelming support.
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