Greek bus drivers and migrant passengers seeking to reach western Europe blocked traffic on the main highway running north to Former Yugoslav Republic of Macedonia (FYROM) on Thursday, tempers fraying over the slow pace of passage over the border.
At least 80 buses have been backed up short of Greece's northern border with FYROM for several days.
FYROM is letting migrants cross only in small numbers, blaming a protest by FYROM taxi drivers who have parked their cars on the railway line because they are angry that police have been directing migrants first to trains and buses for the journey north to Serbia en route to western Europe.
One train left the FYROM border station of Gevgelija on Wednesday morning carrying hundreds of migrants, amid reports of a deal with the taxi drivers, but by the afternoon they had resumed their protest.
On Thursday, after another night spent at a petrol station 14 km (8.7 miles) south of the border, frustrated Greek bus drivers parked their vehicles across the main road and were joined by hundreds of migrants chanting “Merkel, Merkel”, a Reuters cameraman at the scene said.
Most migrants, many of them refugees from Syria, are heading for Germany where Chancellor Angela Merkel is facing a rising public backlash against her decision last year to open Germany's doors to those fleeing increasing armed conflict and deprivation in the Middle East, Africa and Asia.
“There's a small flow towards Skopje and it's causing tension among migrants who have been waiting for days,” a Greek police source said on condition of anonymity.
Another Greek official said: “Authorities in Skopje are allowing a few migrants to pass, and because of a boost in arrivals there are migrants waiting in the cold for days.”
Balkan states straddling the migrant route northwards to wealthy western and northern Europe have begun denying passage to all those not coming from conflict regions of Syria, Iraq or Afghanistan.
More than 1.1 million people fleeing poverty, war and repression in the Middle East, Asia and Africa reached Europe's shores last year, almost all heading for Germany.
More than 62,000 arrived in Greece last month, and a further 91,671 in Germany.