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.
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.
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 four years ago.
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.
A different scene greeted the monarch hours later when he was driven around the Grand Palace, where supporters had waited for hours to catch a glimpse.
King Vajiralongkorn and Queen Suthida waved from the car to a cheering crowd of around 200 people all dressed in yellow, the royal colour.
- Elite backlash -
The ultra-wealthy king sits at the apex of Thai power with the support of 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.
Their demands have prompted a backlash from the kingdom's political elite, and more than two dozen activists and student leaders have been arrested, charged with sedition, and released on bail since July.
But the movement has soldiered on.
After the arrests at Democracy Monument, a crowd of about 300 marched on the city's police headquarters to call for authorities to "release our friends".
"Tomorrow will be a historic day. If anyone wants to...see us kick out a dictator, please come," said Penguin, wearing a Captain America T-shirt, shouted in reference to Premier Prayut -- who masterminded the country's most recent coup in 2014.
Bangkok police say 14,000 officers will be deployed to monitor Wednesday's protest.