Tens of thousands of Danish teachers have been barred from schools after talks over working hours with municipal authorities failed to produce an agreement.
The National Teachers Union says 52,000 teachers were locked out, preventing hundreds of thousands of pupils from attending school.
Thousands of teachers held impromptu protests throughout Denmark.
The union has refused to accept a new working agreement that is part of sweeping school reform programme. Municipal authorities are demanding longer school days, which teachers are resisting.
The central government could intervene and force teachers back into classes if the two sides are unable to reach an agreement.
Municipal authorities want the right to say how much time teachers spend in the classroom, and reject the teachers union's demand for a cap of 25 hours per week in class, with the rest of their working hours used for preparation. Teachers currently spend on average 16 hours a week in the classroom.
"We are prepared to spend more time teaching in the classroom, but we believe there must be a limit," said union executive member Gordon Orskov Madesen. "We also need proper time for preparation to offer quality in each and every lesson."
Michael Ziegler, chief negotiator for Local Government Denmark, an association of municipalities, said local governments should have more say in how teachers spend their working hours.