Thai police have used tear gas at an anti-government protest in the capital Bangkok, the scene of several outbreaks of violent unrest in recent years.
Police fired 10 tear gas canisters at a group of demonstrators who tried to break through a roadblock to get to a rally site.
"So far tear gas was used in one area because protesters did not comply with the rules," said national police spokesman Major General Piya Uthayo on Saturday. "No casualties were reported."
"We tried to solve the confrontation peacefully but had to resort to using tear gas when they refused to retreat," Uthayo told the Spring News television station.
Thousands of police have been deployed for the rally at the Royal Plaza, which is organised by the royalist group Pitak Siam, which opposes Prime Minister Yingluck Shinawatra's government.
Yingluck accused the demonstrators of seeking to overthrow the government and warned of possible violence.
Turn out expected in mass
The authorities expect tens of thousands of people to attend the demonstration, the first major street protest against Yingluck's 16-month-old administration.
Police estimated that about 10,000 protesters were gathered by about 9:00 am.
The Pitak Siam group hopes to attract at least 50,000 protesters outside Parliament Saturday. It claims the government of Prime Minister Yingluck Shinawatra is against the monarchy and corrupt.
The government has invoked a special security law, the Internal Security Act, in three districts of the capital to cope with possible unrest.
Yingluck on Thursday voiced fears the protesters aimed to use violence and to "overthrow an elected government and democratic rule", in a televised address to the nation.
Politically turbulent Thailand has been rocked by a series of sometimes violent rival street protests in recent years, although an uneasy calm has returned after national elections in 2011.
Two months of mass opposition protests in 2010 by "Red Shirt" supporters of ousted premier Thaksin Shinawatra sparked a deadly military crackdown that left about 90 people dead and nearly 1,900 wounded.