Thai police fired tear gas and detained dozens of demonstrators as clashes erupted during the first major street protests against Prime Minister Yingluck Shinawatra's government.
About 17,000 police on Saturday were deployed for the rally in Bangkok's historic district, which was organised by the royalist group Pitak Siam, a relatively new force in the kingdom's fractured political scene.
"In the name of Pitak Siam and its allies I promise that we will topple this government," the movement's head, retired general Boonlert Kaewprasit, declared from the rally stage.
The authorities expected tens of thousands of people to attend the demo, which comes about two and a half years after dozens of people died in a military crackdown on opposition protests in the heart of the capital.
Police estimated that about 20,000 people participated in the rally.
Police said they fired tear gas at a group of protesters who removed barbed wire and barriers blocking their route in front of a UN building close to the main rally site.
"Tear gas was used in one area because protesters did not comply with the rules," said national police spokesman Major General Piya Uthayo.
The authorities called in an extra 5,700 police after the skirmishes, but said they would allow the rally to go ahead at the Royal Plaza so long as the other protesters gathered peacefully.
The demonstrators, who included supporters of the influential "Yellow Shirt" royalist movement, called on Yingluck's government to stand down.
"I can't stand that they disrespect the king. I want the government to quit," said 48-year-old farmer Namsai Jantarat from the northern province of Chiang Mai.
Yingluck accused the demonstrators - led by royalist group Pitak Siam (or "Protect Thailand") - of seeking to overthrow the government and warned of possible violence at the protest on Saturday. The rally is meant to continue through the end of the weekend.
Police used tear gas on Saturday morning at the anti-government protest in Bangkok, the scene of several outbreaks of violent unrest in recent years.
The 10 tear gas canisters were fired 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.
"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.
So far, 37 people have been treated for injures, officials said, and 132 have been arrested. The Bangkok Post reported that 11 people had been taken to the hospital because of the tear gas - including five police officers.
Al Jazeera's Wayne Hay reports from Bangkok that the situation "is still very tense" on the fringes of the protest, which is ringed by security forces.
The government has invoked a special security law, the Internal Security Act, in three districts of the capital to cope with possible unrest.
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.
"It seems that Thailand just goes through this cycle of a few years of calm … and then sooner or later the violence and the street protests start again," said Al Jazeera's Wayne Hay. "Some fear that we are entering a new wave of political unrest right now."
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.
Pitak Siam, by contrast, is linked to the "Yellow Shirt" protest movement, whose rallies in 2006 led to the coup that ousted Thaksin.
"At this stage, the government is more a threat to itself. If it overreacts using an army of policemen that's going to enrage demonstrators and things could get out of control," said Thitinan Pongsudhirak, a political analyst at Bangkok's Chulalongkorn University.
"The use of tear gas is a bad omen and conjures images of the 2008 anti-government protesters who forced their way into government house and parliament."