Thai police shot water cannon on Saturday at anti-government protesters from behind a barricade of shipping containers and barbed wire to stop protesters from advancing towards the Grand Palace in Bangkok.
About 1,000 protesters gathered near the Grand Palace in Bangkok in an area known as Sanam Luang, or Royal Field.
"We have prepared officers to maintain order," police deputy spokesman Kissana Pattanacharoen told reporters earlier, but declined to say how many officers were deployed.
Public gatherings violated laws to contain the spread of coronavirus, he said.
Police with riot shields were seen moving into the area.
The rally comes after parliament this week failed to pass a bill to rewrite the military-backed constitution, one of the protesters' main demands.
A mass trial for protest leaders began this week against activists who were accused of sedition and insulting the monarchy.
Activists on Saturday flew kites spray-painted with images of jailed protest leaders to call for their release.
Thailand's youth protest movement has posed the biggest challenge so far to Prime Minster Prayuth Chan-ocha. Protesters say he engineered a process that would preserve the political status quo and keep him in power after a 2019 election. Prayuth has rejected that.
Protesters also broke a traditional taboo by demanding reform of the monarchy, saying the constitution drafted by the military after the 2014 coup gives the king too much power.