The drama began in the wee hours of yesterday, when leaders of Thailand's anti-government protests sent out urgent messages on Facebook and Twitter calling for demonstrators to gather in Bangkok at 8am instead of 2pm.

They said they had heard that certain groups would try to occupy the space around Bangkok's Democracy Monument, so they decided to move their plans forward to thwart any sabotage attempt.

This raised fears of a clash with royalist groups planning to mass at key sections near the monument yesterday morning wearing the colour yellow in a show of support for King Maha Vajiralongkorn. The monarch, who keeps a home in Germany, was due to pass by last night on his way to a royal temple.

One of the protest leaders, Mr Arnon Nampa, promised that demonstrators would not block the motorcade but would flash the three-finger anti-dictatorship salute as it passed.

Mr Arnon was the first protest leader to raise the topic of monarchy reform; he did so on stage in August and the act earned him both threats and plaudits in a country where insulting the monarchy is punishable by up to 15 years in jail.

Since then, protesters have repeatedly called for the monarch's power and expenditure to be trimmed.

The royalists yesterday were joined by men with uniform crew cuts, dressed in yellow polo shirts, who filed into the space around the Democracy Monument and sat on the pavements before 9am.

The towering edifice, which sits in the middle of a traffic circle in the grand Ratchadamnoen Avenue, was built to commemorate the 1932 revolution that ended absolute monarchy. In recent times, city officials have barricaded it and filled the interior with potted plants.

Small confrontations between royalist and anti-government protesters fizzled out just as quickly as they started.

As the afternoon sun took hold, protesters began removing the plants from the monument in a symbolic act of reclaiming the space.

High-school students in school uniforms and grey-haired women formed a human chain, passing the pots of greenery from one person to the next until the sidewalks were blanketed with plants.

Then protest leaders announced that they would begin their march to Government House, to press the ruling powers to amend the Constitution and hold fresh elections.

After they left, city cleaners swooped in to freshen up the streets in preparation for the royal motorcade. People in yellow clothing sat on yellow mats on the sidewalks, waiting to pay their respects to the monarch.

The marching protesters took a detour, avoiding a major road on which the royal motorcade was expected to pass.

But a commotion was reported near Government House as the police pushed back protesters to let a car carrying Queen Suthida through. Protesters flashed the three-finger salute as it passed.

In the end, the royal motorcade managed to pass largely unhindered through Ratchadamnoen Avenue yesterday evening.

As the royalists dispersed and the road was reopened to traffic, the Democracy Monument remained shorn of plants.