Police in Bangkok deployed and used water cannons against thousands of demonstrators marching through Bangkok on Sunday.
Reuters news agency estimated a strong crowd of over 10,000 people taking part in a march towards 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.
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.
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.
Thailand’s student-led democracy movement began in July, they 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.