The buildup 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 Slovenia and Croatia, both EU members, and Serbia and Macedonia said they would restrict the number of daily arrivals 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.
By Saturday, some 5,500 people were stranded at Idomeni, local police said, with another 800 gathered at another provisional camp some 20 kilometres (12 miles) away.
Since Thursday night, only 150 people have crossed into Macedonia, officials said, putting the total number of refugees and migrants currently on Greek soil at around 25,000.
Tensions were running high along the border with some 400 people protesting by the Idomeni crossing early on Saturday, demanding that the Macedonian authorities let them in.
Friday's announcement by four Balkan states came a week after Austria said it would only allow 80 people claim asylum per day, and also pledged to limit the daily number of people crossing the country to 3,200.
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.
Austria has repeatedly accused Greece of failing to police its borders properly and allowing an excessively-high number of migrants to continue their journey northwards to western Europe.
"Europe can't be indifferent to those of our partners who don't respect their obligations," Greek President Prokopis Pavlopoulos said on Saturday, referring to the border restrictions.
Last week, the EU told Austria that limiting asylum claims was "plainly incompatible" with European and international laws.
'Open the borders'
On Saturday, around 200 people held a protest outside the Austrian embassy in Athens, an AFP correspondent said.
Chanting slogans like "Open the borders" and "Stop the War" in Syria, which has sent millions into exile, they demanded safe passage for refugees.
In a bid to regulate the flow of refugees until the border situation is resolved, Greek authorities are trying to house them on the islands where they land by boat from neighbouring Turkey.
On Saturday morning, another 437 people landed in Piraeus port, Athens News Agency said.