Police have used water cannon against thousands of demonstrators marching through Bangkok on Sunday.
The group - estimated to be as many as 10,000 people by Reuters news agency - were marching to the royal palace to give messages to King Maha Vajiralongkorn.
Officers used the cannon for only the second time during the months-long protests.
Demonstrators have been demanding government reforms since July.
They demand the resignation of Prime Minister Prayuth Chan-ocha - a former general who seized power in a 2014 coup - as well as changes to the constitution and an end to the harassment of state critics.
Controversially, the protesters have also questioned the powers of the monarchy. Thailand's lèse-majesté law, which forbids the insult of the monarchy, is among the strictest in the world.
Many of those marching on Sunday brought handwritten letters addressed to the king calling for reforms and for the monarchy to stop interfering in politics.
Officers used the water cannon against the protesters as they struggled to get through a barricade blocking their route to the Grand Palace from the Democracy Monument in the capital.
A police spokesman however said officers had only fired the cannon "at the sky" and did not intend to harm anyone, according to Bangkok-based news site Khaosod.
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.