Thailand's anti-government protesters are blockading buildings where ballot papers are being stored, two days before the general election.
At least one office in Bangkok has been surrounded and several in southern Thailand in an attempt to prevent ballot papers being distributed.
The protesters oppose the poll, which is sure to be won by the ruling party.
They want the government replaced by an unelected "people's council" to reform the political system.
Prime Minister Yingluck Shinawatra called the 2 February polls in response to the protests - but the opposition are boycotting them. Voting could well be disrupted and there are fears of violence.
Because of disruption to candidate registration, the elections will also not deliver enough MPs for a quorum in parliament, meaning that by-elections will be needed before a government can be approved, extending the instability.
Last week, there were chaotic scenes as protesters tried to stop advance voters from casting their ballots.
Protest leader Suthep Thaugsuban, a former deputy prime minister under a previous opposition-led government, said election day polling would not be blocked.
"Those who want to vote should go and vote," he said on Thursday. "We won't block you from voting otherwise you'll turn around and say we violated your rights."
But some protesters are already surrounding post offices and other buildings where ballot papers are being kept, preventing them from being distributed to polling stations, reports the BBC's Jonathan Head in Bangkok.
One election commissioner has predicted that 10% of polling stations will not be able to open at all on Sunday, our correspondent says.
The army says it will increase the number of troops deployed in Bangkok for the polls on Sunday. Some 10,000 police will also be on the streets.
"In addition to the 5,000 soldiers we have already deployed in and around Bangkok to help monitor security, we will be increasing troops around protest sites as there are people trying to instigate violence," army spokesman Winthai Suvaree told Reuters news agency earlier this week.
At least 10 people have been killed since the anti-government campaign began late last year. So far the government "red-shirt" supporters have mostly stayed off the streets, but observers fear a trigger that caused them to protest would spark more violence.
The protests began in November, after the lower house backed a controversial amnesty bill that critics said would allow Ms Yingluck's brother, Thaksin Shinawatra, to return.
Mr Thaksin was ousted as prime minister by the military in a 2006 coup. He was convicted in absentia of corruption and lives overseas, but the protesters say he controls Ms Yingluck's government.
They also accuse her Pheu Thai party and its Thaksin-allied predecessors - which have won the last five elections - of misusing state funds on ill-judged schemes that win rural votes.
The protesters say they want Thailand's political system reformed and an end to money politics.
Ms Yingluck, however, who leads an elected government that enjoys strong support in rural areas, has asked protesters to respect Thailand's democratic principles.
Her brother, Mr Thaksin, is a deeply polarising figure loved by the rural poor but despised by the urban elite. The divisions between these sides have led to political instability in Thailand for almost a decade.
Correspondents have described it as a raw power struggle between the governing party, and the groups now allied with the protesters and Thailand's royalist establishment.
Each side fears the other will monopolise control of the state and marginalise its opponents, they say.
The election is seen as unlikely to resolve the crisis. The last election that was boycotted by the opposition was annulled.
Both Ms Yingluck and lawmakers from the ruling party also face investigations by Thailand's anti-corruption commission - paving the way for judicial moves against them. Pro-Thaksin governments have been forced from power by legal rulings in the past.