Their Puea Thai Party seems likely to win any new election, which the government says must be held on Feb. 2 now parliament has been dissolved and the date endorsed by the king.
However, a member of the Election Commission said on Saturday the vote could be held on May 4, arguing that was permissible under the constitution because candidates had been prevented from registering in some districts, meaning there would be no quorum to open parliament after a February poll.
Eight people, including two police officers, have been killed and scores injured in violence between protesters, police and government supporters in recent weeks, although there has been no sustained fighting between rival groups.
Police said seven people were wounded in the early hours of Saturday when gunmen on motorcycles fired at anti-government protesters in central Bangkok near the Khao San Road area, which is popular with tourists.
(Read more: In Thailand, rallying cry is against too much democracy)
The government will deploy 10,000 police to maintain law and order on Monday, along with 8,000 soldiers who will be used mainly to protect government buildings.
Fears of more clashes between rival factions escalated after pro-government "red shirts" said they would begin their own rallies from Sunday in provinces neighbouring Bangkok and in a northeastern stronghold, Udon Thani, where leaders said they expected 10,000 people by Monday.
Many Thais believe the military could step in to break the political deadlock, especially if the protests turn more violent, and as rumors of a coup grow.
The army has staged or attempted 18 coups in 81 years of on-off democracy but has tried to remain neutral this time.
Army chief Prayuth Chan-ocha said on Saturday he feared an escalation in violence next week, adding: "I want to tell all sides they must not clash with each other ... We are all Thais and can live together despite our differences."
The protesters say they will block seven main intersections in Bangkok, causing gridlock in a city clogged with traffic at the best of times, and they could shut off other roads as part of a prolonged siege of the city.
Foreign Minister Surapong Tovichakchaikul said on Friday it was alarmist of the U.S. embassy to advise its citizens to get in two weeks' supply of food and water.
While some residents have been stocking up on food and other essentials, retailers have not experienced the kind of panic buying seen, for example, during floods that threatened to overwhelm Bangkok in 2011.
However, the unrest has hurt tourist arrivals and further delayed the start of huge infrastructure projects that had been expected to support the economy this year at a time when exports remain weak. Consumer confidence is at a two-year low.
City officials have instructed 140 schools to close on Monday and some universities near the seven main protest sites have suspended classes from Monday to Wednesday.