Thai pro-democracy protesters said Saturday they would defy an emergency decree banning gatherings for a third consecutive day after confrontations the previous night saw riot police use water cannons against the mainly young activists.

About 3,000 demonstrators in the city's main shopping mall district demanded the release of arrested protesters, and some shouted obscenities against Prime Minister Prayut Chan-O-Cha before they were dispersed by police.

Free Youth, one of the main protest groups, Saturday morning called on its supporters to return to the streets later Saturday.

"Be prepared both physically and mentally for the demonstration and to cope with a crackdown if it happens," the group's online post said.

Several hundred riot police dispersed the protesters Friday by firing blue-dyed water laced with a chemical agent from the cannons.

Police later said the dye would mark protesters for future legal action.

The youth-led movement has jolted the kingdom's political elite with its demands for the government to step down and reforms of the previously unassailable monarchy.

The ultra-wealthy and powerful King Maha Vajiralongkorn has not addressed the civil unrest directly but during a ceremony broadcast Friday reminded people that Thailand "needs people who love the country, people who love the institution of the monarchy".

Pro-democracy protesters have been calling for the resignation of Premier Prayut Chan-O-Cha and government reforms
Pro-democracy protesters have been calling for the resignation of Premier Prayut Chan-O-Cha and government reforms AFP / Lillian SUWANRUMPHA

Thai Lawyers for Human Rights told AFP that 65 activists had been arrested since Tuesday, including nine of the most prominent figures of the pro-democracy movement.

Earlier on Friday, two activists were arrested under a rarely used law banning violence against the queen because they were among a group surrounding a royal motorcade on Wednesday during a large demonstration.

Both men could face life in prison if convicted.

A Thai journalist was also briefly detained during Friday's protest, according to his online news outlet, Prachathai.

Premier Prayut has extended the emergency decree banning gatherings of more than four people until mid-November, ignoring activists' calls for him to resign.

The former army chief was the mastermind behind a 2014 coup before being voted into power in last year's election which protesters say was rigged in his favour.

Opposition party Pheu Thai has called on the government to lift emergency measures and free those detained.

Human Rights Watch on Saturday warned the emergency decree gave police "the green light to commit rights abuses with impunity" and called for international condemnation.