Amid chaotic scenes in Croatia, crowds of migrants briefly broke through riot police lines at the border with Serbia.
There was a further lack of control at Bezdan, the northernmost border crossing between Serbia and Croatia, where large groups of migrants pushed through police lines, after hours of frustration.
Correspondents described the scenes in Tovarnik as mayhem, after several thousand migrants who had crossed from Serbia were held back by riot police, trying to get them to wait for transport on from the border.
Croatia said it would allow migrants to travel into northern Europe, but officials in Slovenia - which borders Croatia to the north - have said they would tighten their border security and would stick to rules which require asylum-seekers to register on arrival.
On Wednesday, hundreds were involved in clashes at the Hungary-Serbia border after Hungarian authorities used water cannon and tear gas to stop migrants forcing their way through.
