Tens of thousands of protesters defied a ban on gatherings of more than four people to mass at a major Bangkok landmark Sunday, carrying posters portraying activists detained in four straight days of strident anti-government rallies.

The youth-led movement has suffered several blows this week, with scores arrested after demonstrators surrounded a royal motorcade and flashed "democracy salutes" at Queen Suthida.

A Thai protester jokingly uses a traffic cone as a loudhailer during a rally at Victory Monument in Bangkok. The protestors are calling for the resignation of the prime minister and reforms to the monarchy
A Thai protester jokingly uses a traffic cone as a loudhailer during a rally at Victory Monument in Bangkok. The protestors are calling for the resignation of the prime minister and reforms to the monarchy AFP / Jack TAYLOR

The government reacted with emergency measures -- including banning gatherings of more than four people in Bangkok -- and the arrest of protest leaders who have called for the removal of Prime Minister Prayut Chan-O-Cha, a former military chief brought to power in a 2014 coup.

Police also used water cannon on demonstrators in Bangkok's central shopping district Friday in an escalation of tactics that sparked outrage across Thai society.

An estimated 20,000 protesters gathered near Victory Monument in Bangkok despite a ban on public gatherings
An estimated 20,000 protesters gathered near Victory Monument in Bangkok despite a ban on public gatherings AFP / Mladen ANTONOV

But the crackdown has served only to embolden the movement's mostly young supporters, who have turned up in large numbers to rallying points announced at short notice in a bid to outwit authorities.

"I cannot let the students fight alone," said 24-year-old Phat, a first-time rallygoer at Bangkok's Victory Monument.

A street vendor sells food as protesters take part in an anti-government rally at Asok in Bangkok
A street vendor sells food as protesters take part in an anti-government rally at Asok in Bangkok AFP / Mladen ANTONOV

Premier Prayut -- the object of the movement's ire -- said protesters must "exercise their rights under the law", but worries there may be players with bad intentions seeking to "create violence", said his spokesman Anucha Burapachaisri.

"The Government repeats that it is ready to listen to all people," he said in a statement.

The police maintained a low-key presence Sunday as local media said more than 20,000 people descended on the landmark from late afternoon shouting "Free our friends" while carrying posters of those arrested.

Thai pro-democracy protesters hold up portraits of arrested activists at Bangkok's Victory Monument in the fourth consecutive day of demonstrations in defiance of an emergency decree banning gatherings.
Thai pro-democracy protesters hold up portraits of arrested activists at Bangkok's Victory Monument in the fourth consecutive day of demonstrations in defiance of an emergency decree banning gatherings. AFPTV / Jonathan KLEIN
A Buddhist monk gives the three-finger salute with pro-democracy protesters during a rally at Asok in Bangkok
A Buddhist monk gives the three-finger salute with pro-democracy protesters during a rally at Asok in Bangkok AFP / Mladen ANTONOV

Those portrayed included human rights lawyer Anon Numpa, Parit "Penguin" Chiwarak, and Panusaya Sithijirawattanakul -- three of the most recognisable faces in the movement and who have consistently called for reforms to the monarchy.

Among their demands is the abolition of a draconian royal defamation law -- which shields King Maha Vajiralongkorn from criticism -- and a call for the monarch to stay out of the country's turbulent politics.

Victory Monument, one of Bangkok's busiest thoroughfares, was blocked off by protesters
Victory Monument, one of Bangkok's busiest thoroughfares, was blocked off by protesters AFP / Jack TAYLOR

Once-taboo in Thailand, the demands for royal reform is one of the biggest challenges facing the kingdom's conservative military-aligned government.

"There are groups of people claiming the monarchy for their own benefit and to get rid of their political opponents," said a 24-year-old graduate who asked not to be identified.

"We will not get true democracy if there's no monarchy reform," he told AFP.

Pro-democracy protesters hold up their mobile phones flashlights as they listen to a speaker during a rally at Bangkok's Victory Monument
Pro-democracy protesters hold up their mobile phones flashlights as they listen to a speaker during a rally at Bangkok's Victory Monument AFP / Lillian SUWANRUMPHA

The establishment elite are "unlikely to budge", said analyst Thitinan Pongsudhirak of Chulalongkorn University.

"Meeting any demand can be slippery and lead to other sweeping changes," he told AFP, adding that "tension and confrontation" will likely build as the daily protests continue.

But the persistently large turnout for the guerrilla demonstrations have now proved the protesters "are a force to be reckoned with", Thitinan said.

Victory Monument, one of Bangkok's busiest thoroughfares, was blocked off by protesters, who cleared the path for emergency vehicles and sent supplies down human chains formed along streets leading to the roundabout.

Since the movement started in July, the social media-savvy protesters have harnessed unorthodox ways of spreading their messages, sending alerts through newly formed groups on Telegram -- a secure messaging app -- and borrowing tips from Hong Kong's pro-democracy protests.

Another protest site in Asok, a popular shopping and restaurant district, drew a smaller group of protesters who practiced hand signals to warn others of an impending crackdown -- wary of a repeat of Friday's clashes.

"They were just kids with empty hands," said 65-year-old Suk, upset by the use of water cannons against protesters. "All they had were their mobile phones."

By 8:00 pm, organisers announced an end to Bangkok's protests, and the crowd slowly dispersed.

Gatherings were planned across the country -- from Phuket in the south to Khon Kaen in the northeast, where students held up a portrait of Prayut with the words "Get out" scrawled on it.

The group Thai Lawyers for Human Rights reported that at least 80 people have been arrested since Tuesday.

So far, charges against the protesters have ranged from breaking emergency measures to sedition.

Authorities defended their use of force Sunday, saying it was according to "international standards".