By Aukkarapon Niyomyat and Amy Sawitta Lefevre
BANGKOK (Reuters) - Thai police fired tear gas at stone-throwing demonstrators and made dozens of arrests on Saturday as thousands demanded the overthrow of Prime Minister Yingluck Shinawatra in the biggest street protest yet against her 16-month-old government.
Protesters from the royalist Pitak Siam group, led by retired military general Boonlert Kaewprasit, repeatedly tried to breach police lines in the Thai capital and rammed a six-wheel truck into a security barrier but were held back.
They accused Yingluck's government of corruption, being a puppet of former premier Thaksin Shinawatra, Yingluck's brother, and of disloyalty to Thailand's monarchy. Many were members of a yellow-shirt protest movement that helped trigger a coup by royalist generals in 2006 that toppled Thaksin.
"If I can't overthrow this government, I am prepared to die," Boonlert told supporters. He made a plea for "soldiers to come and protect us" shortly before ending the protest as torrential rain fell in early evening.
The clashes revived memories of a tumultuous 2008, when yellow-shirted protesters seized government offices, fought street battles with police, and occupied Bangkok's main airports for eight days. Amid that turbulence, two pro-Thaksin prime ministers were forced to resign by the courts.
Yingluck won a 2011 election by a landslide on support from the rural and urban poor. Her supporters, who wear red shirts at protests, held mass street rallies in 2010 against a military-backed Democrat-led government. Those demonstrations ended with a bloody army crackdown in which 91 people were killed.
"Our biggest concern is if Pitak Siam decides to escalate their rally or protesters move into key government buildings including parliament," said Piya Uthayo, a national police spokesman.
Pitak Siam, or Defend Siam — an old name for Thailand -- taps many of the same supporters who backed the yellow shirts: the traditional Bangkok elite that includes generals, royal advisers, middle-class bureaucrats and old-money families.
Saturday's rally follows a visit to Thailand by U.S. President Barack Obama -- part of a three-country tour of Asia -- during which he praised Yingluck, calling her a "democratically-elected Prime Minister who is committed to democracy."
"CORRUPTED AND CRUEL GOVERNMENT"
Thaksin, a billionaire former telecoms tycoon, remains a divisive figure, revered by the rural poor and reviled by Bangkok's elite. He fled Thailand in 2008 and was convicted in absentia of abuse of power -- charges he says were politically motivated. He now lives in Dubai but remains influential.
The confrontation started in the morning when about 500 protesters tried to breach police barricades at Saphan Makawan Bridge and the Misakawan intersection to make their way to a large plaza where about 15,000 protesters gathered. They ignored police who told them to pass through a designated entry point.
At least 42 people were wounded, including seven police, according to the Erawan Medical Center which monitors Bangkok hospitals. Up to 132 protesters were arrested, police said. Knives and bullets were seized from some protesters, they added.
Authorities deployed 17,000 police at the rally site after the government invoked the Internal Security Act allowing police to detain protesters and carry out security checks and set up roadblocks.
The protest highlights tensions that have been simmering since Yingluck's Puea Thai party swept to victory in July 2011.
"The use of tear gas is a bad omen and conjures images of the 2008 anti-government protesters who forced their way into government house and parliament," said Thitinan Pongsudhirak, a political analyst at Chulalongkorn University in Bangkok.
Protesters held signs with pictures of Yingluck including one bearing the caption "PM=Puppet Moron" as others waved yellow flags associated with the royal family.
"I'm telling Thaksin that if he wants to return to Thailand, he needs to bow before the king and serve his prison sentence," Boonlert told the demonstrators. "The world will see this corrupted and cruel government. The world can see the government under a puppet."
Thailand has seen a series of protests since 2006 with pro-Thaksin and anti-Thaksin groups taking turns to challenge various administrations' right to rule.
(Editing by Jason Szep and Sanjeev Miglani)