Thai police on Sunday fired water cannon on pro-democracy protesters who were trying to march to the royal palace to deliver letters demanding reform to the unassailable monarchy.

Student-led rallies have rocked the kingdom since July, with protesters calling for the removal of Premier Prayut Chan-O-Cha -- a former army chief who came into power through a 2014 coup -- and a rewrite to the military-scripted constitution.

The boldest in the leaderless movement have also demanded reforms to the monarchy -- a once-taboo issue -- sending shockwaves through Thailand's arch-royalist establishment.

On Sunday, nearly 7,000 protesters -- a police estimate -- attempted to march from Bangkok's historic Democracy Monument to the Grand Palace to deliver letters to King Maha Vajiralongkorn, with security forces calling for them to stop.

As the marchers got closer, police briefly fired water cannon at them. The protesters reacted angrily, shouting: "Why did you use the water cannons against us?"

Authorities had warned earlier in the day that protesters were banned from breaching a 150-metre radius around the palace, and some 9,000 officials were deployed.

Police defended their brief use of water cannon late Sunday night, saying it was just "a warning".

Sunday's confrontation was the second time Thai police have used water cannon against protesters
Sunday's confrontation was the second time Thai police have used water cannon against protesters AFP / Mladen ANTONOV

"Authorities had no intention to cause any harm," deputy police spokesman Colonel Kissana Phathanacharoen said in a press briefing, adding that only "clean water" was used.

Sunday's confrontation was the second time police have deployed water cannon against protesters.

Last month, a peaceful rally in downtown Bangkok saw protesters blasted with chemical-laced liquid as police bore down -- images that shocked many in Thailand.

Since then, demonstrators have appeared prepared for any retaliation from authorities, bringing along umbrellas, goggles, and even hard hats.

The movement has also borrowed tactics from Hong Kong's pro-democracy protesters, who shared tips on Twitter using trending hashtags on how to react to police crackdowns.

Pro-democracy protests have rocked Thailand since July
Pro-democracy protests have rocked Thailand since July AFP / Jack TAYLOR

The youth-led movement's direct challenge to the monarchy is unprecedented in Thailand, where King Maha Vajiralongkorn sits at the apex of power supported by a coup-happy military and its billionaire clans.

Among the students' demands are the abolition of a royal defamation law -- which shields the monarch from criticism -- a clear accounting of the palace's finances, and for the king to "stay out" of Thailand's turbulent politics.

The king last week addressed a reporter's question on the non-stop protests across the capital, declaring "love" for all Thais and saying that "Thailand is the land of compromise".

But a protest leader Sunday said the use of water cannon on marchers was not acceptable.

"We came to submit our letters," Jutatip Sirikhan told reporters after the confrontation.

"No matter what, we will not step back," she said as protesters chanted "slaves to dictatorship" at security forces who watched on.

Organisers had brought along crimson-coloured homemade mailboxes -- with the words "Royal Household Bureau" emblazoned across them -- for protesters to submit their letters to the king.

Police allowed them to leave the mailboxes outside the Supreme Court, and most protesters dispersed around 9 pm (1400 GMT).

Before the march to the palace, the rally at the Democracy Monument was peaceful with protesters chanting "Prayut, get out" and holding up a three-finger salute -- a symbol of the movement.

They also threw flowers into a makeshift cardboard coffin carrying a life-sized mannequin with Prayut's face. Some scrawled messages on it, including "go to hell".

Earlier Sunday, prominent pro-democracy figures called on the king to listen to the protesters.

"We hope you will change your behaviour once and for all and become a King of all people," wrote lawyer Anon Numpa, one of the most recognisable faces in the movement.

"I hope Your Majesty will open your mind and reach out to dialogue with us to solve the crises together."

The unprecedented demands to Thailand's ultra-wealthy monarch have infuriated pro-royalist groups, and they have retaliated with counter-rallies.

On Sunday, a smaller group of the king's supporters came out to the Democracy Monument holding portraits of King Vajiralongkorn, but they left after they were outnumbered by the protesters.

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