Over 40 people were left injured after clashes between protesters and police in Thailand’s capital Bangkok.
Tuesday’s protest is said to be the most violent since the student-led movement emerged in July.
Protesters were attempting to reach parliament, where lawmakers were debating possible changes to the constitution.
Chaos erupted after a group of demonstrators tried to cut through razor-wire barricades near parliament. They hurled smoke bombs and bags of paint at lines of riot police.
In response, police used a water cannon to try to force them back and when that failed, they used the cannon to fire a liquid laced with a tear-gas solution.
Health officials said five people had been treated in hospital for the effects of tear gas, while others were treated at the scene.
Some protesters suffered gunshot wounds, though police denied using live rounds or rubber bullets during the operation, according to news agencies AFP and Reuters.
The proposed constitutional changes have been one of the core demands of the months-long protest movement.
Others included reforms to the monarchy and the removal of Prime Minister Prayuth Chan-ocha – a former general who seized power in a 2014 coup.
Legislators are expected to vote on whether to accept any of the proposed changes by Wednesday evening local time.
The reforms that the protesters are calling for could make Thailand’s King Maha Vajiralongkorn more accountable and also reform the senate, where the members are unelected.
How protests began?
Thailand’s student-led democracy movement began in July, they have been calling for the resignation of Prime Minister Prayuth Chan-ocha. The former army chief had seized power in a 2014 coup before he was appointed as premier after controversial elections last year.
The protests have widened in recent months to also call for curbs on the king’s powers.
Demonstrators also demand the constitution be rewritten; that the authorities stop harassing critics and royal reform are particularly sensitive in where criticism of the monarchy is punishable by long prison sentences.
Royalists have come out to oppose the student-led demonstrations, and authorities have tried to crack down on the movement.
They believe the protesters want the abolition of the monarchy, something they deny.