Thousands of people from all over Thailand are converging on Bangkok for a rally aimed at toppling the government of Prime Minister Abhisit Vejjajhiva.
Many are backers of former Prime Minister Thaksin Shinawatra, who was ousted in a 2006 military coup.
Dressed in red shirts, his supporters have been waving flags and shouting slogans as they prepare for the rally Sunday.
Mr. Thaksin currently lives in Dubai but is still popular among Thailand's poor and rural population. His supporters have been demanding new elections, which they hope will return him to power.
Security forces have been stopping trucks carrying protesters to search for weapons.
Organizers say between 600,000 and 700,000 supporters of the opposition United Front for Democracy Against Dictatorship (UDD) will march in Bangkok. But they say the protest will be peaceful.
Authorities have deployed thousands of police and troops to maintain order.
The last opposition protest in April 2009 deteriorated into a riot which left two people dead and many others injured. There was also considerable damage.
Recent polls show that a large segment of the Thai population is fed up with protests, which have hurt their country's lucrative tourism industry.
Mr. Thaksin left the country to avoid a 2-year prison sentence on corruption charges.
Prime Minister Abhisit is backed by Thailand's upper class.
Some information for this report was provided by AP.