BERLIN (AP) — Climate activists protested at two German airports early Thursday, causing numerous flights to be canceled during the peak holiday travel period.

The group Last Generation said several of its members entered the grounds of Hamburg airport around 6 a.m. and glued themselves to the runway. Thursday is the first day of school vacation in the northern state.

“It can't yet be predicted when operations can resume,” the airport operators said. “According to current information about a dozen flights have had to be canceled. Further cancelations and diversions aren't ruled out."

Members of the group also cut through a security fence at Duesseldorf airport, in the west of the country, and blocked an access route to the runway.

Last Generation is known for its disruptive protests, blocking roads and airports to demand tougher government action to reduce greenhouse gas emissions. German prosecutors raided the homes of several of the group's members in May on suspicion of forming or supporting a criminal organization.