Police in Macedonia have fired tear gas at thousands of migrants trying to enter from Greece.
It comes a day after Macedonia declared a state of emergency in two border regions to cope with an influx of migrants, many from the Middle East.
Macedonia has become a major transit point for migrants trying to reach northern EU members.
Some 44,000 people have reportedly travelled through Macedonia in the past two months.
At least four people were injured in the clashes at the border with Greece, AP reported.
Greece itself has seen almost 160,000 people landing on its shores since January, the UN estimates, with 50,000 arriving in the past month alone.
There were fears that by effectively shutting off its borders, Macedonia would create a backlog of migrants at its frontiers.