Millions of Thais are voting in a general election boycotted by the opposition and blighted by protests.Anti-government protesters are trying to disrupt the vote across Thailand and continue their campaign to force Prime Minister Yingluck Shinawatra to resign.
Ms 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.
“The situation overall is calm and we haven’t received any reports of violence this morning,” National Security Council chief Paradorn Pattanatabutr told Reuters.
“The protesters are rallying peacefully to show their opposition to this election,” he said.
Security officials said about 130,000 personnel would be deployed across Thailand on Sunday, including 12,000 in Bangkok.
Polling stations opened at 08:00 (01:00 GMT) and will close at 15:00 (08:00 GMT), but there has been little campaigning and it is unclear how many Thais will turn out.
Polling is being disrupted in at least three districts in the capital and elsewhere in the south, strongholds of the opposition Democrat Party, which is boycotting the election.
The BBC’s Jonathan Head in Bangkok says crowds of demonstrators there are blocking access to would-be voters at some polling stations and preventing ballot papers reaching polling stations.
“Today is an important day,'” Ms Yingluck said as she voted. “I would like to invite Thai people to come out and vote to uphold democracy.”
Some voters expressed frustration when they found their local polling stations blockaded by opposition supporters.
“This is too much. I want to vote,”‘ 42-year-old Yupin Pintong told the Associated Press news agency. “I don’t care if there’s violence. I will be really upset if I don’t get to vote.”
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,” Mr Nachtwey told the Wall Street Journal.
Correspondents say less unrest is expected in the rural north and east, because Ms Yingluck’s Pheu Thai party has such overwhelming support.
The Democrat Party, which is allied to the protesters, has been unable to win a majority in parliament for more than two decades.
Many of their members want the government to be replaced by an unelected “people’s council” that would oversee wide reform of the political system.
Even if the election passes off peacefully, the result will almost certainly be challenged in the courts and could be annulled.
Ms Yingluck’s party is already facing a host of legal challenges that could see it disbanded, as has happened with pro-Thaksin parties in the past.