BANGKOK (AP) — The first major demonstration this year by Thailand’s student-led pro-democracy movement on Wednesday turned into a confrontation with police, who quickly arrested 10 activists who were putting up banners and posters.

The rally by more than 1,000 people in central Bangkok was called by the Ratsadorn movement, which campaigned last year for Prime Minister Prayuth Chan-ocha and his government to step down, the constitution to be amended and the reform of the monarchy to make it more accountable.

Wednesday’s rally focused on the detention of four top protest leaders who were charged Tuesday with defaming the monarchy, or lese majeste. The movement has been seeking the abolition of the lese majeste law, which carries a punishment of three to 15 years in prison on each count.

“Today, we are gathering here to start our first fight after the system of injustice took away our friends,” said Panupong Jadnok, a protest leader.

The rally began tensely when police arrested 10 activists who were putting up banners and signs in the public space. They were charged with littering.

After several speeches, protest leaders led the crowd to the police station where the activists were being held to demand their release.

At least 100 police faced off against the protesters when a tear gas canister exploded. An angered crowd scattered and then pushed back against the police, but calm was restored after a few minutes.

The origin of the tear gas canister was unclear. Police denied throwing it. The crowd dispersed after the 10 activists were processed by police and released.