BANGKOK (AP) — Thailand has invoked a special security law for an anti-government rally Saturday that is expected to be the largest since Prime Minister Yingluck Shinawatra took office in 2011. Thousands of police officers will be deployed at the rally, which officials fear could turn violent.

Authorities expect tens of thousands of protesters, a turnout that would serve as a sharp reminder of the deep political tensions in the country despite two years of relative calm.

The demonstration is being organized by a royalist group calling itself "Pitak Siam" — or "Protect Thailand" — at Bangkok’s Royal Plaza, a public space near Parliament that has been used by protesters in the past.

Led by retired army Gen. Boonlert Kaewprasit, the group accuses Yingluck’s government of corruption, ignoring insults to the revered monarchy and being a puppet of former Prime Minister Thaksin Shinawatra. Thaksin, Yingluck’s brother, was ousted by a 2006 coup that sent the country into spasms of political upheaval.

While the group is a newcomer to Thailand’s protest scene, it is linked to the infamous "Yellow Shirt" protesters, whose rallies led to Thaksin’s overthrow. They later toppled a Thaksin ally after invading and shutting Bangkok’s two airports for a week in 2008.

Yingluck is 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.

"If a large number of people are mobilized by incitement … and there is evidence that violence may be used to achieve those ends, then this is a case of national security," Yingluck said in her nationally televised address.

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.

National police chief spokesman Maj. Gen. Piya Uthayo said Friday that nearly 17,000 police officers had been called in from around the country to provide security for the rally.

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.