As frustrations boiled over at restrictions imposed on people moving through the Balkans, migrants trapped on the Greece-Macedonia border tore down a metal gate in the barbed wire fence.
A Reuters witness said Macedonian police fired several rounds of teargas into the crowd and onto a railway line where other migrants sat refusing to move, demanding to cross into the country.
Greece raced to set up temporary accommodation for a build-up of thousands of migrants stranded in the country after Austria and countries along the Balkans migration route imposed restrictions on their borders, limiting the number of migrants able to cross.
Many of the migrants, fleeing war and poverty in the Middle East and North Africa, hope to reach Germany, which last year took in 1.1 million asylum seekers.
There were an estimated 22,000 migrants and refugees trapped in Greece on February 29, some sleeping rough in central Athens, some in an abandoned airport and at the 2004 Olympic Games venues.
Greece's migration minister said without any outlet, that figure could rise as high as 70,000 in coming days.
More than 1 million migrants passed through the country last year, prompting criticism from other European nations that Athens was simply waving them through.
"These people do not want to stay here," said Thodoris Dritsas, Greece's shipping minister. "Even if we had a system in place for them to stay here permanently it wouldn't work."