THESSALONIKI, Greece (AP) — Police in northern Greece have arrested six people and detained at least 40 others following clashes involving youths backed by an extreme right-wing political group.

The youths entered a vocational high school and hurled gasoline bombs, flares and rocks at a rival gathering organized by left-wing university student groups against government education reforms.

The clashes occurred Wednesday in Thessaloniki, Greece’s second-largest city, where police used tear gas to break up the violent demonstration as classes at the school were ongoing.

A youth wing of the extreme right group Golden Dawn had earlier expressed support for the counter-demonstration.

A Greek court ruled last year that the far-right party was operating as a criminal organization, and sentenced members of its leadership including former lawmakers to up to 13 years in prison.