Croatia has closed seven of its eight road border crossings withSerbia following a huge influx of migrants.
Officials said they had no choice after more than 13,000 people entered the country since Hungary fenced off its border with Serbia earlier this week.
Many have been taken by bus to reception centres but some say they plan to walk to neighbouring Slovenia.
Huge numbers of people heading north from the Mediterranean have created a political crisis in the European Union.
Croatian officials said roads leading to the border crossings had also been shut.
The crossing on the main road linking Belgrade and Zagreb - at Bajakovo - appeared to be the only one left open.
Local media reported severe congestion at the crossing, with a 6km queue of lorries back into Serbia.
In other developments:
- A migrant thought to be Syrian is electrocuted at the entrance to the Eurotunnel in Calais
- Hungarian Prime Minister Viktor Orban says Hungary has started building a fence along part of its border with Croatia, after media reports that migrants were crossing it