Thai police have fired tear gas and detained dozens of demonstrators as clashes erupted at the first major street protests against Prime Minister Yingluck Shinawatra's government.
The violence, while relatively small scale, appeared to mark a new phase in Thailand's long-running political crisis pitting Thai royalists against ousted premier Thaksin Shinawatra - Yingluck's brother - and his supporters.
About 17,000 police 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 demo comes two and a half years after dozens of people died in a military crackdown on "Red Shirt" protests against the previous government in the heart of the capital.
About 20,000 people were estimated to have joined Saturday's rally, according to police, far short of the half a million organisers had hoped would attend.
Tensions flared as police fired several rounds of tear gas at protesters trying to ram through barriers near the main rally site in the Royal Plaza using a truck.
"Tear gas was used in one area because protesters did not comply with the rules," said national police spokesman Major General Piya Uthayo.
About 100 protesters were detained while knives and bullets were confiscated, he said.
Thirty-seven people, including a number of police officers, were treated for cuts and other injuries, officials said.
The authorities called in an extra 5,700 police after the clashes, but said they would allow the rally to go ahead at the Royal Plaza so long as the other protesters gathered peacefully.
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.
The government invoked the Internal Security Act (ISA) in three districts of the capital to cope with possible unrest, giving the police additional powers to block routes, impose a curfew, ban gatherings and carry out searches.
"We will evaluate the situation daily and if it escalates we are ready to invoke emergency rule," Thai police chief General Adul Sangsingkaew said on national television.
Thailand has been rocked by a series of sometimes violent rival street protests in recent years, although an uneasy calm had returned after national elections in 2011.
Two months of mass opposition protests in 2010 by Red Shirt supporters loyal to Thaksin - who was toppled in a coup in 2006 - sparked a deadly military crackdown that left about 90 people dead and nearly 1,900 wounded.
At their height, those rallies drew about 100,000 people demanding the resignation of the previous government.
Thaksin's sister Yingluck is now prime minister after his political allies won a landslide election victory last year.
Thaksin, who made billions as a telecoms tycoon, is adored by many poor Thais for his populist policies while in power, but reviled by many in elite, military and palace circles who see him as authoritarian and a threat to the monarchy.
The demonstrators at Saturday's rally, 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.
Observers say prosecutions - often targeting people linked to the Red Shirts - for insulting the monarchy have surged since royalist generals overthrew Thaksin following a series of mass protests by the rival Yellows.
The Red Shirts threatened to strike back in the event of a new coup.
"This rally is illegitimate," Red Shirt leader Thida Thavornseth said at a news conference on Saturday. "We will come out in force if there is any sign of a coup or the government loses control."