Climate activists protested at two German airports early on Thursday, causing numerous flights to be cancelled during the peak holiday travel period.
The group Last Generation said several of its members entered the grounds of Hamburg Airport at around 6am and glued themselves to the runway.
Thursday is the first day of school holidays 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 cancelled. Further cancellations 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 organisation.