The build-up began in earnest last week after Macedonia began refusing entry to Afghans and imposed stricter document controls on Syrians and Iraqis, slowing the passage of migrants and refugees to a trickle.
And the situation looked set to worsen significantly after EU members Slovenia and Croatia, as well as Serbia and Macedonia said they would limit the number of migrants entering each day to 580.
The tighter controls have left thousands -- including many children -- stranded in Greece, as the bloc's worst migration crisis since World War II shows no sign of abating.
Macedonian authorities opened the Idomeni border crossing today afternoon and dozens of migrants were allowed to pass through, with local Greek police sources saying 300 were expected to cross the frontier by the end of the day.
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More than 20,000 refugees and other migrants are currently on Greek soil, according to the office of migration minister Yiannis Mouzalas.
Tensions had run high along the border earlier in the day, with some 400 migrants staging a protest demanding entry to Macedonia.
The move has sparked a bitter spat between Athens and Vienna, with Greece fearful it would trigger a domino effect, leaving thousands stranded on its territory after landing on its beaches from Turkey.
Austria has repeatedly accused Greece of failing to police its borders properly and allowing an excessively high number of migrants to continue their journey towards northern Europe.
"Europe can't be indifferent to those of our partners who don't respect their obligations," Greek President Prokopis Pavlopoulos said of the border restrictions today.