The extent of disruptions was not immediately clear when polls opened nationwide, but there were early indications that dozens of polling stations in Bangkok would not open because protesters blocked delivery of ballots or stopped voters from entering.
At least seven people were wounded in Saturday's clashes, including an American photojournalist, when gun battles broke out between government supporters and protesters intent on derailing the polls.
Protesters, led by the People's Democratic Reform Committee (PDRC), say they plan to fill the streets of the capital to prevent voters from reaching polling stations.
"How did we get to this point?" asked Chanida Pakdeebanchasak, a 28-year-old Bangkok resident who was determined to cast her ballot Sunday no matter what happens. "Since when does going to vote mean you don't love the country?"
Although unrest has already hit Bangkok and polling stations are not expected to open in some parts of the South, voting was expected to proceed smoothly in most of the country.
Police said they will deploy 100,000 officers nationwide, while the army is putting 5,000 soldiers into Bangkok to boost security. More than 47 million people are registered to vote.
Since protests began late last year, at least 10 people have been killed and nearly 600 wounded.