Police arrested 21 protesters Tuesday during a Bangkok rally that saw dozens of people give a royal motorcade a three-fingered salute -- a show of defiance by Thai pro-democracy activists calling for reforms to the monarchy.

Since July, Thailand has seen frequent youth-led demonstrations demanding the resignation of Premier Prayut Chan-O-Cha, a former army chief.

Some in the leaderless movement have also called for reform to the monarchy -- a once-taboo topic due to Thailand's harsh royal defamation laws.

Some Thai protesters gave a three-fingered salute as a royal motorcvade passed by, a rare show of defiance
Some Thai protesters gave a three-fingered salute as a royal motorcvade passed by, a rare show of defiance AFP / Lillian SUWANRUMPHA

Tuesday's protest at Democracy Monument -- a major intersection that has been the site of previous demonstrations -- drew dozens of supporters chanting and dancing in front of police.

Authorities moved in to clear the road in the late afternoon for a royal motorcade ferrying King Maha Vajiralongkorn, deputy police spokesman Colonel Kissana Phathanacharoen told AFP.

"Officers tried to negotiate with the protesters who were occupying the area. They were uncooperative," he said.

Some police were splattered with blue paint during the demonstration
Some police were splattered with blue paint during the demonstration AFP / Mladen ANTONOV

Charges against the 21 protesters include obstructing police and "causing disorder", said a police press release.

Demonstrators threw blue paint across the intersection -- with some splashing uniformed officers -- before the leaders were bundled into a van.

"They dragged, beat and pulled us away," prominent activist Jatupat Boonpattarasaksa -- who was among those arrested -- said on a Facebook live broadcast from inside a police truck.

The royal family was scheduled to host the premier at the Grand Palace Tuesday, a public holiday to commemorate the death of the late long-serving King Bhumibol Adulyadej.

Some 21 protesters were arrested during the demonstration in central Bangkok
Some 21 protesters were arrested during the demonstration in central Bangkok AFP / Lillian SUWANRUMPHA

When the royal motorcade eventually passed, dozens held up a three-finger salute -- taken from the popular "Hunger Games" film trilogy and harnessed as a pro-democracy symbol.

The ultra-wealthy King Vajiralongkorn sits at the apex of Thai power, supported by the kingdom's military and billionaire clans.

Some protest leaders have called for the abolition of royal defamation laws -- which shields the monarch from criticism -- and for a proper accounting of the institution's fortune, estimated to be some $60 billion.

Since July, more than two dozen activists and student leaders have been arrested, charged with sedition, and released on bail.

But the movement has soldiered on.

A demonstration in Bangkok last month attracted some 30,000 supporters -- the largest public rally since the 2014 coup masterminded by now-premier Prayut.

On Wednesday, thousands are expected to gather at Democracy Monument for the 47th anniversary of a student uprising which ended the regime of a hated military dictator.

Bangkok's Metropolitan Police have said 14,000 officers will be deployed to monitor the situation.