BANGKOK (AP) — Pro-democracy protesters in Thailand were confronted by riot police and sprayed by water cannons Sunday as they tried to approach Bangkok's Grand Palace to deliver letters about their political grievances addressed to the country's king.

The pro-democracy movement has been pushing a bold challenge to reform the country's monarchy with almost daily demonstrations. Sunday marked the second time water cannons were used against them during several months of demonstrations.

The protesters pushed aside one of several buses that along with barbed wire was serving as a barrier to marchers trying to approach the palace, which houses the royal offices but is only used by King Maha Vajiralongkorn on infrequent ceremonial occasions. The attempt to break through came after police had declared their march illegal and asked for protesters to send representatives to talk.

The protesters had met earlier at Bangkok’s Democracy Monument and marched as darkness fell, pushing past an initial thin line of police.

Protesters threw objects at police during the melee, but both sides backed off after a few minutes, and it appeared that order had been at least temporarily restored.

The water cannons were employed by the police for just a short time, and it appeared that no one suffered any serious injuries.

“People just wanted to submit the letters. There was no sign of violence from protesters at all," said protester Thawatchai Tongsuk, 36. “If the police gave way, I believe that the leaders would have submitted the letters and then been finished. Everyone would go home.”

“The more violence they use, the more people will join the protest,” Thawatchai said.

The demonstrators had solicited letters to the king from protest supporters that were deposited in mock red postboxes on wheels that marchers said they intended to deliver. The action was the latest gimmick by the protest movement to maintain public interest in their cause.

The student-led movement, which over several months has seized the political initiative, has put enough pressure on the government of Prime Minister Prayuth Chan-ocha to call for Parliament to deal with at least some of their demands.

They are seeking Prayuth’s resignation, changes to the constitution to make it more democratic and reforms to the monarchy to make it more accountable.

The protesters believe Prayuth lacks legitimacy because he came to power after an election last year whose rules were set up under military rule. Prayuth as army chief in 2014 led a coup ousting an elected government and then headed the junta that ran the country until last year’s polls.

A new constitution was put into effect by the junta that the protesters also consider illegitimate and anti-democratic.

The third demand, calling for reform of the monarchy, is the most controversial. The monarchy has traditionally been an untouchable institution, regarded by most Thais as the heart and soul of the nation. A lese majeste law mandates a prison term of up to 15 years for anyone who defames the king or his close family.

Until the protesters raised the issue, public criticism of the royal institution was virtually unknown.

While the protesters have increasingly put the monarchy issue front and center, they have received serious pushback. Even the main opposition party, otherwise sympathetic to their other points, has said it does not want to amend laws covering the monarchy, and royalists have started holding counter-demonstrations. A few dozen rallied briefly Sunday across from the main protest.

Parliament has agreed to debate amending the constitution and political leaders are discussing setting up a reconciliation committee, an effort that so far has been rejected by the protesters.

But Prayuth has insisted he won’t step down, and any effort to reform the monarchy seems to be a dead end, leaving the situation deadlocked.