Just one day before a major anti-government rally, police on Tuesday afternoon arrested 21 political activists during a swoop on a small demonstration at the Democracy Monument.

There was a chaotic scene as policemen dragged one of the activists, Chatuphat Boonpattharaksa, from the top of a pick-up truck and bundled him into a waiting police van as dozens of demonstrators screamed in protest.

Chatuphat is better known as Pai Doadin who has been staging intermittent anti-government activities since the military coup in 2014.

Twenty others were also taken into custody and immediately brought to the headquarters of Region One Border Patrol Police at Klong Luang, Pathum Thani.

Deputy police spokesman Police Colonel Krissana Pattanacharoen said the activists were charged with organizing unauthorized public gathering and other minor offences, including obstruction of traffic and unauthorized use of loudspeakers.

During the scuffle at the rally site at the Democracy Monument, demonstrators threw paint at the policemen to try to stop them from arresting the activists.

The arrests prompted dozens of members of the so-called Ratsadorn Group,  led by student activist Parit Chiwarak, to rally in front of the headquarters of the national police in Pathumwan to demand an immediate release of the activists.

Police said the confrontation today was sparked by the demonstrators’ refusal to move away from the road to clear the way for a royal motorcade on its way to the Temple of the Emerald Buddha.

Parit, who is better known as Penguin, said the arrests would further fuel public resentment toward the Prayut government and called for a massive turnout for tomorrow’s rally at the Democracy Monument.

At press time, the 21 activists were still being held in police custody while the small group of demonstrators led by Parit dispersed at around 9 pm without any untoward incidents.

The so-called Ratsadorn Gorup styles itself after a group of military men and civilians who led a revolution that ended Thailand’s absolute monarchy in 1932. It hopes to use tomorrow’s rally to pressure Prime Minister Prayut Chan-o-cha to resign and for a new constitutional to be adopted.

The group also wants to press ahead with its demand for a sweeping reform of the monarchy.

Meanwhile, Suthep Thaugsuban, who led the months-long street protests against the Yingluck government that triggered the military coup in 2014 said today he will mobilize people to show up on Rajdamnoen Avenue to greet the royal motorcade of HM the King.

In his Facebook post, Suthep, who is the former leader of the People’s Democratic Reform Committee, urged his supporters to turn up in yellow at Maha Chesadabodin Pavilion, to welcome the HM King’s motorcade and to remain there until the King leaves.