Thai police fired tear gas as tensions flared at an anti-government protest Saturday in the capital Bangkok, the scene of several outbreaks of violent unrest in recent years.

Thousands of police have been deployed for the rally, organized by the royalist group Pitak Siam, which wants Prime Minister Yingluck Shinawatra's government to step down.

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 at the Royal Plaza in the city's historic district.

"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, told demonstrators from the rally stage.

Police fired 10 tear gas canisters 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, police said.

"Tear gas was used in one area because protesters did not comply with the rules," said national police spokesman Major General Piya Uthayo.

Three people, including one police officer, were taken to hospital because of the effects of the tear gas, while several others received first aid at the site, according to the city's Erawan emergency centre.

The authorities said they would allow the rally to go ahead at the Royal Plaza so long as protesters gathered peacefully.

However, later 130 demonstrators were detained, some of them carrying knives and bullets.

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 has invoked a special security law, the Internal Security Act (ISA), in three districts of the capital to cope with possible unrest.

"We will evaluate the situation daily and if it escalates we are ready to invoke emergency rule, but so far I think the ISA will be sufficient," Thai police chief General Adul Sangsingkaew said on national television.

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.

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.

"This government ignores widespread disrespect of the monarchy and even supports the perpetrators. It is a puppet of Thaksin," Pitak Siam spokesman Vachara Riddhagni told AFP ahead of the demo.

Observers say prosecutions for insulting the monarchy have surged since royalist generals toppled Thaksin in a coup in 2006. Many of those targeted are linked to the Red Shirt movement.