BANGKOK -- Police in Bangkok fired tear gas to disperse swarms of anti-government protesters who began gathering Saturday for a rally that was expected to be the biggest since Thai Prime Minister Yingluck Shinawatra took office last year.
Yingluck had ordered more than 15,000 police into the streets and invoked a special security law this week ahead of the rally, which was expected to draw tens of thousands of protesters. The prime minister accused the demonstrators of seeking to overthrow the government and warned of possible violence.
The demonstration was being organized by a royalist group calling itself "Pitak Siam" -- or "Protect Thailand." It serves as a sharp reminder of the deep political divisions that have split the country since the army toppled Yingluck's brother Thaksin Shinawatra in a 2006 military coup.
The focal point of Saturday's rally was Bangkok's Royal Plaza, a public space near Parliament that has been used by protesters in the past.
Police were allowing protesters into the site, and two roads leading to it were open. But in an effort to control access, security forces erected concrete barriers on another road leading to Royal Plaza. When between 50 to 100 protesters tried to break through one of the barriers, a contingent of around 500 police fired tear gas and beat them back with batons.
While Pitak Siam is a newcomer to Thailand's protest scene, it is linked to the well-known "Yellow Shirt" protesters, whose rallies led to Thaksin's overthrow. The same movement later toppled a Thaksin-allied elected government after occupying and shutting down Bangkok's two airports for a week in 2008.
Thaksin remains a divisive figure in Thai politics. The Yellow Shirts and their allies say he is personally corrupt and accuse him of seeking to undermine the popular constitutional monarch -- charges Thaksin denies.
Yingluck was taking Saturday's rally seriously. Her Cabinet invoked the Internal Security Act on Thursday in three Bangkok districts around the protest site, and she later addressed the nation to explain the move, citing concerns of violence.
The security act allows authorities to close roads, impose curfews and ban use of electronic devices in designated areas. Measures began taking effect Thursday night, with police closing roads around Yingluck's office, the Government House, and placing extra security at the homes of senior officials, including the prime minister.
In a nationally televised address Thursday, Yingluck said protest leaders "seek to overthrow an elected government and democratic rule ... and there is evidence that violence may be used to achieve those ends."
National police chief spokesman Maj. Gen. Piya Uthayo said Friday that more than 15,000 police officers had been called in from around the country to provide security for the rally.
The new protest group's leader is retired army Gen. Boonlert Kaewprasit, who is best known for his role as president of the Thailand Boxing Association. His name is unfamiliar in the anti-Thaksin protest movement, but his message appears to have resonated with Yellow Shirt supporters who have laid low in recent years after Yingluck's party won the last elections.
Analysts said they did not view the protest as an immediate threat to Yingluck's government, but were watching it closely.
"Anytime you have tens of thousands of people converging, assembling in a central Bangkok location, it becomes a government stability concern," said Thitinan Pongsudhirak, a political scientist at Bangkok's Chulalongkorn University.
But he added: "I think it's a serious concern more than a serious threat."
Thailand has been gripped by bouts of political instability since 2006, with Thaksin's supporters and opponents taking turns to spar over who has the right to rule the country.
The most violent episode came in 2010, when Thaksin's "Red Shirt" supporters led a two-month occupation of central Bangkok to demand the resignation of an anti-Thaksin government. The protests sparked a military crackdown that left at least 91 people dead and more than 1,700 injured.
Thaksin has lived in self-imposed exile since 2008, when he jumped bail to evade a corruption conviction and two-year jail term. He retains huge popularity among the rural poor, who want to see him pardoned and returned to power. But he is reviled by the urban elite and educated middle class, who see him as authoritarian and a threat to the monarchy.
Buoyed by Thaksin's political machine, Yingluck was elected by a landslide victory in August 2011. She initially was criticized for her lack of political experience -- she was an executive in Shinawatra family businesses -- but has won praise for leading the country through one of its longest peaceful periods in recent years.
Associated Press photographer Sakchai Lalitkanjanakul contributed to this report.