Thai police clashed with demonstrators and fired tear gas as about 20,000 protesters called for premier Yingluck Shinawatra to step down. More than 100 people were arrested during the unrest.
An estimated 17,000 officers were deployed for the first major antigovernment protest since Shinawatra was elected last year.
At least 37 people were treated for injuries and 130 were arrested following the clashes, AFP reported.
Thai security forces said that violence erupted when they tried to stop protesters attempting to remove barbed wire and barriers in front of the UN building near the demonstration.
The government has invoked the Internal Security Act in three areas of the city, granting security forces wider powers to block roads, impose curfews, conduct searches and bar people from rallying. That officials vowed that emergency rule would be invoked if the violence escalates.
Pitak Siam ("Protect Thailand"), a Thai royalist movement, organized the protest. The movement previously opposed former Prime Minister Thaksin Shinawatra – the new premier’s brother – and also oppose Yingluck. Thaksin was toppled by the military in a 2006 coup, with his sister elected in a landslide victory in August 2011.
"In the name of Pitak Siam and its allies I promise that we will topple this government," movement leader retired general Boonlert Kaewprasit said, AFP reported.
Yingluck accused the demonstrators of seeking to overthrow the leadership and warned of possible violence at the rally, which is set to continue throughout the weekend.
Over two years ago, several months of opposition protests in the Thai capital left 90 dead and nearly 2,000 wounded after a military crackdown.