Protesters destroyed a shopping centre as their rally came to an end
The UN has demanded an independent inquiry into recent unrest in Thailand, when more than 80 people were killed in clashes between security forces and protesters.
UN rights chief Navi Pillay said the guilty must be held accountable.
Opposition MPs have accused Prime Minister Abhisit Vejjajiva of committing violations by ordering the army to crack down on the protesters.
Their nine-week protest paralysed parts of the capital, Bangkok.
Many of the dead were protesters killed when soldiers moved in to dismantle their fortified camp in the city.
The government has repeatedly blamed the violence on "terrorists" it says infiltrated the red-shirt protester ranks, attacking police and soldiers.
The "red-shirt" protesters arrived in Bangkok on 14 March and occupied key parts of the capital, demanding that the government step down.
Attempts to negotiate a political solution failed and on 19 May Thai troops entered the protesters' sprawling camp to end their rally.
Ms Pillay said an inquiry was needed "to foster longer-term political reconciliation".
"I urge the government to ensure that an independent investigation of recent events be conducted, and all those found responsible for human-rights violations are held to account," she said in a speech in Switzerland.
In response, Thailand's UN envoy Sihasak Phuangketkeow said an independent commission was "being set up".
"The Thai government deeply regrets the loss of lives and injuries that occurred, and is committed to bringing those responsible to account," he said.
Meanwhile, Mr Abhisit came under renewed pressure on Monday as the parliament debated a censure motion against him and several ministers.
The opposition Puea Thai Party, broadly seen as supporting the red-shirts, accused the prime minister of using excessive force.
Although the government has a big enough majority to see off any no-confidence motion, analysts say the televised debate has become a focal point in the battle for public opinion.