BELGRADE, Serbia (AP) — Dozens of Serbian high school students on Monday staged a sit-down protest demanding the release from detention of a fellow student jailed during weekend anti-government protests in the Balkan country.

The protest came as President Aleksandar Vucic reportedly called for the release of those people who were detained on misdemeanor charges, but not those charged with more serious offenses.

The students marched Monday from their school in downtown Belgrade toward the main police station in the Serbian capital, where they sat on the ground, blowing whistles and booing.

Authorities said Monday they detained 18 people following incidents during demonstrations against Vucic on Saturday and Sunday. Protesters on Saturday burst into the state-TV building in Belgrade. More skirmishes with police took place Sunday when protesters surrounded the Serbian presidency during Vucic's news conference.

The incidents were the first during some three months of weekly rallies demanding democratic and media freedoms. Critics accuse Vucic of fostering divisions and hate speech in the country while curbing media and other freedoms.

A former extreme nationalist who now says he wants Serbia to join the European Union, Vucic has denied the allegations that he is an autocrat. He said opposition leaders want to topple him without elections.

"As I said, you can walk as much as you want, that's fine, but violence is not allowed," Vucic said. "Anyone who commits violence will be punished."