Votes are being held in Thailand for 29 parliamentary seats which will provide the first test of support for the new Democrat-led coalition government of prime minister, Abhisit Vejjajiva.
His Democrat Party-led coalition came to power in December and only has a slim majority in parliament.
The previous leadership was forced out by a court ruling and months of anti-government protests.
The court ruling also banned 29 MPs from politics, triggering by-elections across 22 states.
The previous governing party, allied to former PM Thaksin Shinawatra, is hoping to pick up enough seats to weaken the new government's hold on power.
Thailand has intensified security for the by-elections.
"Two police will guard each polling station and we have rapid movement units on standby to handle any unexpected situation," said General Wichian Potphosri, adding that he expected peaceful polls.