Amid chaotic scenes in Croatia, crowds of migrants briefly broke through riot police lines at the border with Serbia.
People waiting for hours in blazing heat at Tovarnik grew frustrated, though the situation is quieter now.
Croatia is struggling to deal with at least 7,300 arrivals since Wednesday morning after Hungary closed its border with Serbia, blocking the previous route into the European Union.
EU leaders will hold an emergency summit next week to discuss the crisis.
Croatian officials say migrants must apply for asylum there or be treated as illegal immigrants.
Authorities seem to be overwhelmed and surprised by the number of migrants – most of whom wish to travel on to other EU countries – and the Croatian interior minister has said the country is “absolutely full”.
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.
Croatian police eventually gave way under pressure. Some of the migrants said they intended to walk to Slovenia.
Many are exhausted with supplies of food and water running low.
All morning several thousand people had waited in the heat hoping a train would take them north on their long journey to Germany.
Around midday a group of young men began to mobilise for a protest.
At first they tried to walk towards Zagreb on the rail line but were turned back by the police.