Croatia has closed seven of its eight road border crossings with Serbia 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.