A water cannon was used by Thai police against thousands of protesters marching to deliver a message to King Maha Vajiralongkorn demanding reforms to curb his monarchy's powers and to remove the government.

In months of largely peaceful protests to call for greater democracy and the departure of Prime Minister Prayuth Chan-Ocha, a former junta leader, it was only the second time the water cannon had been used.

Journalists from Reuters estimated that more than 10,000 protesters marched from central Bangkok's Democracy Monument. The police have placed the number at 7,000.

"Reform or revolution," one placard read.

A barricade of buses and barbed wire had reached the protesters. To stop them pushing forward, police fired a water cannon, but witnesses said some had managed to reach the area known as Sanam Luang-or Royal Field-next to the Grand Palace.

Protesters said they were trying to give the Royal Household Bureau a message.

"We no longer want the monarch to intervene in politics," one of the protest leaders, Jutatip Sirikhan, told Reuters.

One 25-year-old protester, who gave his name only as Keng, said, "Please, King, listen to the people, please. People are unhappy because you let the military have full power and approved their coups. We want reform."

Increasingly, since July, protests have called for reforms to the powerful monarchy, breaking a long-standing taboo against criticism of the institution, punishable by up to 15 years in jail.

The protesters say the monarchy has helped enable decades of military domination of Thailand, most recently by approving the premiership of Prayuth, who seized power in a 2014 coup and kept it after disputed elections last year.