Thai police have fired tear gas in clashes with hundreds of protesters in Bangkok seeking to overthrow the government.
Protests are against the administration of Prime Minister Yingluck Shinawatra.
The protest highlights tensions that have been simmering since Yingluck's Puea Thai party swept to victory in July 2011 and could herald another period of unrest in Thailand.
Anti-riot police wielding plastic shields fired gas cannisters at protesters who tried to climb over cement and barbed wire barriers blocking entry to the rally site.
Police said "between 300 and 400 protesters" clashed with police.
At least seven police were injured and up to 132 protesters arrested in the clash near the United Nations headquarters in Bangkok.
Pitak Siam, a new anti-government group, has attracted the support of various royalist groups including "yellow shirt" members of the People's Alliance for Democracy.
PAD helped destabilise governments either led or backed by former premier Thaksin Shinawatra, Yingluck's brother, in 2006 and 2008.
Authorities have deployed 17,000 police at the rally site and the government has invoked the Internal Security Act allowing police to detain protesters and carry out security checks and set up roadblocks.
Police say they have seized various weapons, including knives and bullets, as protesters arrived at the protest area.
Thailand has seen frequent bloody street protests in recent years including a rally that lasted more than two months by supporters of the present government in 2010.
Those protests sparked a military crackdown that left at least 91 people dead and more than 1,700 injured.
The royalist Pitak Siam group, led by retired military general Boonlert Kaewprasit, accused Yingluck's government of corruption and being a puppet of former premier Thaksin.
Thaksin remains a deeply divisive figure in Thailand. He was ousted in a 2006 military-backed coup and fled the country in 2008 shortly before being found guilty of abuse of power.
Some held pictures of Thailand's revered King Bhumibol Adulyadej as Boonlert shouted "Yingluck, get out!" to cheers of his supporters.
Thailand has seen a series of political protests since 2006 with pro-Thaksin and anti-Thaksin groups taking turns to challenge various administrations' right to rule.