Anti-government demonstrators blocked polling stations in Bangkok and several southern provinces Sunday, preventing voters from casting advance ballots a week ahead of the scheduled general elections.
Advance voting for the February 2 election was prevented by demonstrators who surrounded dozens of polling stations and padlocked at least two stations in the capital.
By 9 am, at least 13 polling districts in Bangkok had cancelled voting and shut down because officials and voters could not get in.
The poll disruptions were seen as a serious setback to the government of caretaker Prime Minister Yingluck Shinawatra, which scheduled the snap election to bolster its legitimacy in the face of protests in Bangkok.
Her Puea Thai Party is expected to win the elections because of its popularity in populous northern and north-eastern provinces, and the main opposition Democrat Party has refused to participate.
With electoral boycotts in many southern districts, there are doubts over whether the process will elect enough members to parliament for a necessary quorum to form legal government.
Anti-government protesters are demanding political reform before any new elections, to reduce the continuing influence of Yingluck's elder brother, former premier Thaksin, who fled the country in 2008 to dodge a two-year jail term for corruption.