Renewed clashes have broken out near Thailand’s parliament in the capital, Bangkok, as lawmakers debate possible changes to the constitution.
Police used water cannon and a tear-gas solution against protesters calling for reforms to the monarchy and the military-backed government.
Fighting also broke out between pro-reformists and supporters of the Thai royal family.
The proposed constitutional amendments come after months of protests.
They could make Thailand’s King Maha Vajiralongkorn more accountable and also reform the senate, where the members are unelected.
Protesters are also demanding the resignation of Prime Minister Prayuth Chan-ocha – a former general who seized power in a 2014 coup.
Tuesday’s violence started when 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.
Demonstrators could be seen trying to wash the irritant from their eyes.
Health officials said five people had been treated in hospital for the effects of tear gas, while others had been treated at the scene.
Some protesters tried to shelter behind giant inflatable rubber ducks, which they had intended to float down the river behind parliament as lawmakers debated inside.
Amid the chaos, anti-government protesters then clashed with pro-monarchy supporters, with the rival groups hurling objects at each other.
Police intervened to keep the two groups apart.
Thailand has suffered political turmoil for years but tensions have escalated since protesters began questioning the powers of the monarchy. Thailand’s lèse-majesté law, which forbids any insult to the monarchy, is among the strictest in the world.
“Amending the constitution is going to lead to the abolition of the monarchy,” pro-royalist leader Warong Dechgitvigrom told reporters on Tuesday.
However, protesters deny wanting the abolition of the monarchy.