The rally could herald another period of unrest in Thailand, which has seen frequent bloody street protests in recent years, including a rally lasting more than two months by supporters of the present government in 2010, which was put down by the army.

At least 50,000 protesters are expected to gather near parliament in the capital, Bangkok, on Saturday, Police Chief Adul Saengsingkaew told a news conference.

The government's decision to invoke the Internal Security Act (ISA) in the area of the protest site will allow the police to ban gatherings at sensitive government locations and gives them more power to arrest people on public order grounds.

"This ISA is not in place to disperse the rally or stop it from happening but to make sure the rally takes place peacefully. It is a protective measure to make sure the demonstration runs smoothly," Adul said.

In a televised statement on Thursday, Prime Minister Yingluck Shinawatra said that in invoking the law, she had acted to protect democracy.

"If a large number of people are mobilised by incitement, led by those who seek to overthrow an elected government and democratic rule .... and there is evidence that violence may be used to achieve those ends, then this is a case of national security," Yingluck said.

The group behind the protest, Pitak Siam, is led by retired general Boonlert Kaewprasit. The group held a first rally on October 28, attracting 7,000 supporters according to the police but 20,000 according to the organisers.

It says Yingluck heads a puppet government controlled by her brother, former premier Thaksin Shinawatra, from self-imposed exile in Dubai. He was ousted by the military in 2006 and fled the country in 2008 shortly before being found guilty of abuse of power.

Thaksin remains a divisive figure in Thailand, with opponents accusing him of corruption and republicanism, even though both he and Yingluck have expressed support for the constitutional monarchy.

Thaksin is adored by the poor because of health and welfare policies during his time as prime minister.

Yingluck's Puea Thai party swept to victory in a general election in July 2011 and the country has since enjoyed a period of relative calm, even though it remains politically fractured between the pro- and anti-Thaksin camps.

The rally by Pitak Siam - pitak means "protect" and Siam is Thailand's old name - begins a day before the opposition opens a censure debate against Yingluck and her government, accusing them of corruption and incompetence.

(Reporting by Aukkarapon Niyomyat; Writing by Amy Sawitta Lefevre; Editing by Alan Raybould and Robert Birsel)

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