Long lines of migrants, many of them refugees from Syria, snaked through southern Serbia by foot on Monday before jumping on trains and buses north to Hungary and the last leg of an increasingly desperate journey to western Europe.
State authorities and aid agencies threw up tents and scrambled to supply food and water to thousands surging through the western Balkans, their numbers swelling since Greece began ferrying migrants from overwhelmed islands to the mainland.
Visiting the border between Greece and Former Yugoslav Republic of Macedonia (FYROM), Austrian Foreign Minister Sebastian Kurz said the situation in the Balkans was "dramatic".
"We urgently need coordinated action across Europe," he told ORF radio.
In Serbia, Red Cross official Ahmet Halimi said 8,000 migrants had registered in the southern town of Presevo over the past 24 hours.
Many had spent three desperate days on Greece's northern border after FYROM halted their passage saying it could take no more. But on Saturday, crowds braved batons and stun grenades to storm through police lines.
Helpless to stem the tide, FYROM rushed trains and buses to the border to carry them north, where they crossed into Serbia on foot.
More arrived on Monday, walking from the border crossing of Miratovac some 5 km (3 miles) to a reception centre in Presevo, where many received medical aid, food and papers legalizing their transit through the country.
Most carried their belongings in rucksacks. Men carried small children on their shoulders, hats shielding their heads from the August sun. After a chilly night, daytime temperatures were expected to near 30 degrees Celsius (86°F).
"I just want to cross to continue my journey," said Ahmed, from Syria, on the Serbian border. "My final destination is Germany, hopefully."
Not since the wars of Yugoslavia's collapse in the 1990s has the cash-strapped western Balkans seen such large movements of people, when many Bosnians, Croats, Albanians and Serbs displaced by fighting fled for the rich countries of Europe – the likes of Germany, Austria and Sweden.
The problem threatens to get worse as EU member Hungary, part of Europe's borderless Schengen zone, races to complete a fence along its 175-km border with Serbia to keep the migrants out, threatening to create a dangerous bottleneck.
The European Union, the migrants' only destination, has struggled to formulate and implement a common policy.
Kurz criticised the Greeks, whose borders form part of the EU's external frontier, for failing to process asylum requests on Greek soil as per EU rules. A record 50,000 migrants, many of them Syrians crossing by boat from Turkey, hit Greek shores in July, straining the resources of a country going through one of the worst economic crises of modern times.
On Monday, two people drowned and five were believed missing when a dinghy carrying migrants capsized off the Greek island of Lesbos, where aid groups say 1,500 have been arriving daily for the past week.
The Greek government has chartered a car ferry to collect them from the islands and bring them to the mainland on a daily basis. It carried 2,500 people, mainly Syrians, on Monday to Athens, where buses awaited to take them north.
"It is not just a case of Greece not processing those (asylum) claims, but they are actively doing their very best to get the refugees to move on to central Europe as soon as possible," Kurz said.
"The Western Balkan countries are overrun, overwhelmed and have been left to their own devices," Austria's APA news agency quoted Kurz as saying. "We have to help them."
Germany says it expects a record 750,000 asylum-seekers to arrive this year, in a crisis overwhelming authorities in Europe from the Greek islands to the French port of Calais.
Fleeing war and poverty in the Middle East, Africa and Asia, many have undertaken dangerous journeys across sea and land in search of safety and employment in the affluent nations of western and northern Europe.
The UNHCR urged the EU to do more. The problem, said UNHCR Europe Bureau Director Vincent Cochetel, "will not go away any time soon and affects all of Europe."
In the past two weeks, over 23,000 have entered Serbia, taking the total so far this year to some 90,000.