The government has vowed to push ahead with the general election, despite threats by anti-government protesters, camped out at major intersections in Bangkok, that they will disrupt the polls in an attempt to stop Prime Minister Yingluck Shinawatra's Puea Thai Party from returning to power.
The anti-government protesters took to the streets in November in the latest round of an eight-year conflict that pits Bangkok's middle class, southern Thais and the royalist establishment against the mostly poor, rural supporters of Ms Yingluck and her brother, former premier Thaksin Shinawatra, who was ousted in 2006 but who is said to still have huge influence.
The main opposition Democrat party, which backs the anti-government protests, is boycotting the election, which Ms Yingluck's party is bound to win but without enough members to achieve a quorum in parliament.
The prospect of polling stations having to close early because of trouble on the streets will only add to doubts about the vote's legitimacy.
Puchong Nutrawong, secretary-general of the Election Commission, said it was concentrating on security in Bangkok and the south, where the opposition was strong.
The protesters, members of the People's Democratic Reform Committee, forced polling stations in 49 of 50 districts in Bangkok to shut last weekend and voting could only go ahead in three of 15 southern provinces.
Mr Puchong said: "I've asked commission officials to call polling venues in southern Thailand today to ensure we are as prepared as we can be. If any polling station faces a security threat it can shut down."
Protesters have threatened to obstruct access to polling stations, although protest leader Suthep Thaugsuban said his supporters would not stop people voting.