At least 41 people were hurt, some with gunshot wounds, when demonstrators marching on the Thai parliament clashed with police and royalist counter-protesters, in the worst violence since a new youth-led protest movement emerged in July.
Police fired water cannon and teargas at protesters who cut through razor-wire barricades and removed concrete barriers outside parliament.
The police denied that they had opened fire with live ammunition or rubber bullets, and said they were investigating who might have used firearms.
The protest movement, which has called for deep constitutional reform to a system demonstrators say has entrenched the power of the military, has emerged as the biggest challenge to Thailand's establishment in years.
Thousands of demonstrators converged on parliament to put pressure on lawmakers discussing changes to the constitution.
The protesters also want the removal of Prime Minister Prayuth Chan-ocha, a former army ruler, and to curb the powers of King Maha Vajiralongkorn.
Bangkok's Erawan Medical Centre said at least 41 people were hurt.
It did not immediately give a full breakdown, but said at least 12 were suffering from teargas and five people had gunshot wounds.
Thai police used water cannon with chemical-laced water to disperse protesters during a pro-democracy demonstration Source: Sipa USA Varuth Pongsapipatt / SOPA Image
It did not say who might have used firearms.
"We tried to avoid clashes," the deputy head of Bangkok police, Piya Tavichai, told a news conference.
He said police had tried to push back protesters from parliament and to separate them and the yellow-shirted royalist counter-protesters.
Protesters advanced on police with makeshift shields, including inflatable pool ducks.
After about six hours, police pulled back and abandoned their water trucks, which the protesters mounted and sprayed with graffiti.
Pro-democracy protesters take cover with inflatable ducks and umbrellas as police fire water cannon Source: AP
"I hereby announce the escalation of the protests. We will not give in. There will be no compromise," Parit "Penguin" Chiwarak told the crowd at the gates of parliament before protesters dispersed.
Another protest was set for central Bangkok on Wednesday.
Government spokesman Anucha Burapachaisri said police had been obliged to use teargas and water cannon to keep parliamentarians safe.
Prime Minister Prayuth took power as the head of a military junta in 2014 and remained in office after an election last year.
Pro-democracy protesters fight with riot-policemen during a rally near the Parliament in Bangkok, Thailand Source: Sipa USA Chaiwat Subprasom / SOPA Images/
He rejects opposition accusations that the election was unfair.
Politicians were discussing several proposals for constitutional changes, most of which would exclude the possibility of altering the monarchy's role.
There is also discussion of the role of the upper house Senate, which was entirely selected by Mr Prayuth's former junta and helped ensure that he kept power with a parliamentary majority after last year's disputed vote.
Some protesters fought with dozens of royalists who had remained behind after an earlier demonstration by hundreds of right-wing Thais calling on lawmakers not to make changes to the constitution.
Pro-democracy protesters take cover as police fire tear gas and water cannon during a demonstration against a charter amendment at Parliament in Bangkok Source: EPA
"Amending the constitution is going to lead to the abolition of the monarchy," royalist leader Warong Dechgitvigrom told reporters.
Protesters have said they do not want to abolish the monarchy.
