Police threw tear gas into a truck carrying demonstrators on Saturday as it attempted to push through a barricade on Ratchdamnoen Avenue leading to the Royal Plaza, where the anti-government protest was planned.

"We tried to solve the confrontation peacefully but had to resort to using tear gas when they refused to retreat," National Police Bureau spokesman Police Major General Piya Uthayo told the Spring News television station.

An estimated 20,000 police have been deployed for the rally at the Royal Plaza which is organized by the pro-royalist group Pitak Siam, which opposes Prime Minister Yingluck Shinawatra's 16-month-old administration.

Led by retired Army General Boonlert Kaewrasit, the group claims the government of Prime Minister Shinawatra is against the monarchy and corrupt.

"The police overreacted and have now harmed the people," Boonlert said in response to the use of tear gas, addressing a swelling crowd outside Parliament.

Ahead of Saturday's rally, officials placed the Royal Plaza district including Parliament, Government House and the United Nations headquarters under the Internal Security Act, which allows authorities to make arrests without charges.

The Pitak Siam movement attracted about 20,000 followers to its first rally on October 28, and police expect tens of thousands of people to attend the demonstration on Saturday.

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.

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.

hc/mr (AFP, dpa)