Thousands of anti-government demonstrators in Bangkok rallied in front of a massive crowd out in force to show support for the Thai king on Wednesday as tensions rose over a burgeoning youth-led protest movement calling for democratic reforms.

Student activists have staged large rallies in recent months demanding the resignation of Premier Prayut Chan-o-cha, a former army chief who first rose to power in a coup six years ago.

Some have also demanded reforms to the kingdom's powerful monarchy -- a move that has prompted a backlash from Thailand's staunchly pro-royalist establishment.

A protest at the capital's Democracy Monument was brought forward by activists worried about potential clashes with royalist groups, who started gathering around the area ahead of a scheduled royal motorcade ferrying King Maha Vajiralongkorn later in the afternoon.

"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. "When the royal motorcade arrives, don't utter swear words."

Most protesters ultimately left the scene and started marching to nearby Government House two hours before the monarch and his wife were due to drive by. Gathered along the motorcade route were thousands of others dressed in yellow shirts -- the king's colours -- who said they were hoping to catch a glimpse of the monarch later.

Some had been ferried into the area in organised trucks and the crowd was massed in numbers that appeared to rival the size of the anti-government demonstrators. Protesters linked arms and marched away chanting "Prayut, get out!" and "Long live the people!" as the royalist group shouted insults at the crowd.

"We want to show that we love the king," said 47-year-old Sirilak Kasemsawat, who accused the pro-democracy movement of wanting to "overthrow" the monarchy.

Activists have repeatedly denied that charge, saying they wish only to "adapt" it for modern times. Their demands include the abolition of a strict royal defamation law -- which shields the king from criticism -- and for the monarch to stay out of the country's turbulent politics.

More than 5,000 anti-government protesters had gathered by afternoon, authorities say, though a count on the ground by AFP reporters estimated double that amount. More than 15,000 police were deployed to control the situation.

'I want a better future'

The gathering comes a day after police clashed with protesters staging an impromptu rally at the same site, arresting 21 activists in an attempt to clear the area for an earlier royal motorcade.

Dozens of remaining protesters raised a three-finger salute as the royal motorcade drove by on Tuesday, a gesture of defiance the pro-democracy movement has borrowed from the popular "Hunger Games" films.

The movement is the latest of several pro-democracy coalitions to appear in the turbulent modern history of Thailand, which has endured long bouts of political unrest and more than a dozen successful military coups.

The army has long positioned itself as the sole defender of the ultra-wealthy king, whose influence permeates every aspect of Thai society. Wednesday's demonstration is intended to commemorate the 47th anniversary of a 1973 student uprising against a hated military dictatorship that saw 77 people killed.

"This could be the last fight for Thailand's democracy," said 18-year-old Attaporn, who travelled from the kingdom's northern Pichit province to join the rally. "I have to do this if I want a better future."

Dozens of anti-government activists have been arrested, charged with sedition, and released on bail since July. And the "grinding confrontation between the anti-establishment movement and royalist-conservative forces" is likely to continue for some time to come, said political analyst Thitinan Pongsudhirak of Chulalongkorn University.

But he added that the student-led movement appeared disorganised and seemed to be struggling to articulate a clear and singular objective.

"The protest movement requires more time and public persuasion that reform and change are needed," he told AFP.