Hundreds of protesters marched on a barracks of Thailand's royal guards unit on Sunday hoisting inflatable rubber ducks high above their heads, a whimsical show of force by a pro-democracy movement calling for curbs to the power of the monarchy.

The yellow toy has been harnessed as a symbol by the protesters, whose leaders have emphasised peaceful tactics despite recent use of water cannon and tear gas by security forces.

But their demands to reform the kingdom's unassailable monarchy -- once a taboo topic due to a draconian royal defamation law -- have sent shockwaves through Thailand's political and ruling class.

Sunday's protest -- the latest in a series of near-daily turnouts across Bangkok -- was meant to target the 11th Infantry Regiment.

Along with the 1st Infantry Regiment, the two units were placed under King Maha Vajiralongkorn's direct control last year -- a move experts saw as a way for the monarch to assert more authority.

"These two regiments have been involved in cracking down on people in the past," said prominent protest leader Parit "Penguin" Chiwarak.

"They have also played key roles in past coups."

The black-clad protesters wore hair clips with yellow rubber ducks, while those on the frontlines donned gas masks and helmets -- prepared for any potential skirmishes with authorities.

Two water cannon trucks were parked at the entrance, which was covered with coils of barbed wire. Police in riot gear stood guard outside.

"The things used against us are bought using our taxes, so they are using our money to hurt us," Farng, a 30-year-old graduate student, told AFP, declining to provide her full name.

"As taxpayers, the army should be serving us, not the monarchy," she said. "Their duty should be to protect the people."

Protesters folded into paper planes a "people's declaration" -- which called for the regiments to be transferred back -- and flew them across the barriers to the stone-faced police.

They also threw red paint at the feet of the police to signify the security forces' role in violent crackdowns on previous pro-democracy movements, before rallygoers dispersed at about 10pm.

Inflatable yellow ducks have been adopted as a symbol by Thai anti-government protesters
Inflatable yellow ducks have been adopted as a symbol by Thai anti-government protesters AFP / Jack TAYLOR

The military has long positioned itself as a defender of Thailand's enormously wealthy royal family, who boast assets conservatively estimated to be some $30-$60 billion.

In the name of protecting the king, the army has staged more than a dozen coups since the end of absolute monarchy in 1932.

The most recent was in 2014.

Coup mastermind General Prayut Chan-O-Cha was head of a junta regime before renewing his lease on power in elections last year, which were governed under a military-scripted constitution.

Besides royal reform, the protesters are also demanding Prayut's removal as premier and a rewrite of the constitution.

So far scores have been charged with sedition, while at least a dozen prominent leaders were summoned for questioning on royal defamation allegations.

The demonstrations have largely been peaceful since the movement started in July.

But earlier this month, at least six people were shot in a chaotic protest that saw police deploy water cannon and tear gas against the youngsters.

It is unclear who opened fire, and officials say they are investigating the incident.

Protesters are undeterred by the step-up in police tactics, with one telling AFP Sunday that the use of force has only galvanised his determination to support the movement.

"The authorities have their weapons, we only have ducks -- it would look bad if they use violence," said Peter, 33, declining to give a full name.

Thailand