Hundreds of refugees on Monday tried to break through a border fence into Macedonia from Greece, where more than 7,000 people are stranded, as anger mounted over travel restrictions on migrants.
In a sign of widening divisions within the European Union, German Chancellor Angela Merkel meanwhile lashed out at Austria and Balkan states for abandoning debt-laden Athens to refugee chaos.
Macedonian police fired tear gas as a group of some 300 Iraqi and Syrians forced their way through a Greek police cordon and raced towards a railway track between the two countries.
"Open the borders!" they shouted as a group of men used a metal signpost to bring down a section of barbed wire fencing into Macedonia, prompting police to fire volleys of tear gas and preventing them from crossing.
At least 30 people, including many children, requested first aid, the charity Doctors of the World said.
The protest occurred several hours after Macedonia allowed just 300 Syrians and Iraqis to cross before resealing the frontier.
With Austria and Balkan states capping the numbers of migrants entering their soil, there has been a swift buildup along the Greece-Macedonia border with Athens warning that the number of people "trapped" could reach up to 70,000 by next month.
As the bottleneck showed little sign of easing, Merkel slammed the restrictions and pointed the finger at Austria, whose introduction of restrictions on February 19 triggered a domino effect in the Balkans.
"We can't just abandon this country," she said, referring to Greece.
The spate of border closures was sparked by Austria's announcement it would accept no more than 80 asylum claims per day and cap the numbers of those seeking to cross its territory.
"Because Austria decided on a limit of 80 per day, and not one more, we have reached today's situation," Merkel said in a TV interview with public broadcaster ARD late Sunday.
"When one insists on his border, the other suffers. That's not my Europe," she said.
The European Commission on Monday said it was working on "contingency plans" to help Greece and other Western Balkan countries cope.
"We are using all available instruments to address emergency needs (...) including the reinforcement of reception capacities, border management, relocation and returns," commission representative Mina Andreeva told reporters in Brussels.
On the ground, thousands continued to mass at the Idomeni crossing on the Greek-Macedonian border, hoping it would be opened after a day of protests in which scores of people lay on railway tracks, some holding slogans reading "Open borders" and "We are humans, not animals".
The build-up at Idomeni camp, which can accomodate up to 1,500 people but is sheltering more than 7,000, began in earnest last week after Macedonia began refusing entry to Afghans and imposed stricter controls on Syrians and Iraqis.
EU members Slovenia and Croatia quickly followed suit along with Serbia, with all four states imposing a daily limit of 580 migrants.
"Do you seriously believe that all the euro states that last year fought all the way to keep Greece in the eurozone -- and we were the strictest -- can one year later allow Greece to, in a way, plunge into chaos?" Merkel said.
But Austria quickly hit back at criticism of its tougher migrant policy, describing it as "absurd".
"We don't have to take criticism from anyone on any side," Interior Minister Johanna Mikl-Leitner told the Austria Press Agency.
"Apparently for some, the European solution (to the crisis) is for all (migrants) to mass in Austria," she said.
Austria, which took in 90,000 asylum-seekers in 2015 and saw almost 10 times as many pass through, last week hosted a migration conference for Balkan countries at which it persuaded them to also impose tougher controls. Neither Greece nor Germany were invited to the talks.
In Athens, ministers were due to hold a special meeting on Monday to forge an "emergency" plan to tackle the problem, Greek media said.
With facilities in the capital nearing breaking point and hundreds of people continuing to arrive at the port of Piraeus, authorities hastily opened two indoor facilities -- one of them a baseball hall disused since the Athens 2004 Olympics -- to shelter 2,000 migrants.
French authorities meanwhile began bulldozing half of the "Jungle" migrant camp in the northern port city of Calais where thousands of migrants and refugees have gathered in the hope of sneaking aboard lorries and ferries to Britain.