Thousands of Thai protesters demanding the prime minister's resignation rallied overnight outside his office in Bangkok Thursday after scuffling with royalists opposed to the youth-led movement's calls for reforms to the monarchy.

Student activists have staged huge demonstrations in recent months calling for Premier Prayut Chan-o-cha, a former army chief who took power in a coup six years ago, to step down.  But some protesters have also demanded reforms to the country's powerful monarchy –- a move that has prompted a backlash from Thailand's staunchly pro-royalist establishment.

Tensions flared near the capital's Democracy Monument ahead of a scheduled afternoon drive-by of a royal motorcade carrying King Maha Vajiralongkorn. Yellow-clad royalists arrived at the venue in numbers to rival the anti-government protesters, forcing police to erect barriers and roadblocks to keep them apart. More than 15,000 police were deployed.

Organisers of the anti-government rally warned supporters not to provoke their opponents. "There will be provocations from the other side so please trust in me -- we don't want to clash with anyone," said Anon Numpa, a prominent activist. Police had cordoned off most of the anti-government protesters away from the royal route, but dozens were still present as the motorcade drove by. 

Queen Suthida could be seen staring from a limousine window as protesters held up three-fingered salutes – a gesture of defiance the pro-democracy movement has borrowed from the popular "Hunger Games" films. 

Such overt challenges to the monarchy are unprecedented in Thailand, where the royal family's influence permeates every aspect of society. "The monarchy has been around more than 700 years," said Sirilak Kasemsawat, one of the thousands of royal supporters waiting "to show we love the king". Brief scuffles with royalists broke out as anti-government protesters made their way to Government House, which was quickly stopped by police intervening. 

Government spokesman Anucha Burapachaisri announced late Wednesday the premier had ordered police to press charges against "the protesters who obstructed the royal motorcade". 

Charges will also be pursued against "those who had acted in a way that defames the monarchy," he said in a statement. 

"They must face legal procedures without exception."

Despite warnings from police to disperse from the Government House after 10 pm (1700 GMT), some camped outside to support the activists, who set up a stage nearby to continue calling for Prayut to "get out".