Thousands of migrants and refugees have effectively been trapped at Greece's northern border after Macedonian officials reportedly banned Afghan nationals from entering the country.
Strict conditions have also been imposed designed to restrict the number of Syrians and Iraqis able to cross the border.
Reports suggest more than 2,000 refugees were camped at the Greek border post in Idomeni by lunchtime on Monday after the new restrictions were introduced at the weekend.
An estimated 61 buses carrying a further 3,000 people were reportedly parked at a petrol station around 12 miles (20km) from the border crossing, awaiting permission from police to continue their trip.
Many have chosen to abandon the buses and make the journey on foot.
A spokeswoman for Greek police's Central Macedonia division has blamed the bottleneck on Serbia, saying that country's government had stopped allowing entry to Afghan nationals.
Macedonia had then imposed the same rules to prevent the refugees from becoming stranded in their country, he added.
Meanwhile, large numbers of people from Syria and Iraq have been refused permission to cross the border as they have lost their documents during their journey, reports claim.