Migrants began crossing from Serbia into Croatia on Wednesday, desperate to find new ways to western Europe after Hungary sealed its frontier and EU states tightened border controls over the unprecedented human influx.
Pressure is building for a special European Union summit to come up with solutions to the crisis, with the bloc bitterly split and its vaunted passport-free Schengen zone in jeopardy, as Germany boosted controls along parts of its frontier with France.
A small group of women and children entered EU member Croatia early on Wednesday, followed shortly afterwards by around 300 mostly Syrians and Afghans in the first of what could be a new surge of migrants seeking to reach the bloc by circumventing the razor-wire barrier erected by Hungary.
Croatia's Prime Minister Zoran Milanovic -- who has denounced blasted Hungary's border fence as "unacceptable" -- said migrants would be given free passage through the country, allowing them to push on towards Slovenia, Austria and Hungary's fenceless southwestern frontier.
"We are ready to accept and direct those people, their religion and colour of skin is completely irrelevant, to where they apparently wish to go -- Germany and Scandinavia," Milanovic told lawmakers.
AFP correspondents saw others heading towards the Croatian frontier -- which is still peppered with minefields from the Balkan wars of the 1990s -- after turning up in buses and taxis at the Serbian border town of Sid.
- 'Hungary is closed' -
"We heard that Hungary was closed so the police told us we should come this way," Amadou, 35, from Mauritania in western Africa, told AFP.
Hundreds more people were trapped Tuesday behind the barrier erected by Hungary along its border with Serbia in an attempt to halt the migrant tide.
Hungary's conservative Prime Minister Viktor Orban is planning a similar structure along the frontier with Romania, prompting outrage from its southern neighbour.
But Budapest insisted the fence was working as it made its first arrests under tough new laws which punish "illegal border-crossing" with prison terms of up to three years.
The controversial measures are part of Orban's strategy to stem the flow of migrants trudging up through the western Balkans, most headed to Germany and Sweden.
The apparent success of the fence in deflecting the flow sparked fears in Serbia that it would be swamped by an unmanageable number of migrants.
Speaking to AFP, Serbia's minister for refugees, Aleksandar Vulin, called on Hungary to reopen its border "at least for women and children" where around a hundred people were waiting in vain to cross.
Hundreds more were stranded in the Turkish border city of Edirne after police stopped around 1,000 refugees from crossing into Greece to continue their journey towards Europe.
And huge crowds were camped out at Istanbul's main bus station for a second night running, after being refused tickets to Edirne.
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"They cannot stay here. Maybe we will allow them to stay two or three days but then they have to leave," Edirne governor Dursun Ali Sahin told Turkey's NTV channel, a day after police surrounded its bus station to prevent the mostly Syrian refugees travelling west towards Greece and Bulgaria.
- Domino effect -
Hungary's hardline anti-migrant stance has been sharply criticised, with the UN refugee agency saying it could be in violation of the 1951 Refugee Convention.
The Council of Europe also said it was "concerned" about its draconian new border law and would press Orban for an explanation.
Romania, which is a member of the EU but not of the passport-free Schengen zone, criticised Hungary's planned border fence as "out of step with the spirit of Europe".
The Hungarian move came as others states moved to reintroduce border controls across Europe, which is grappling with its biggest migratory wave since the end of World War II.
Germany, Austria and Slovakia have introduced identity checks on parts of their borders, and Poland and the Netherlands are considering whether to follow suit.
The measures have caused tailbacks at road crossing points and stoked concern among German road hauliers, who point to rising costs.
- Another blow to Schengen -
Politically, though, the big concern is for the future of the 20-year-old Schengen agreement, considered by many EU supporters as important as the euro.
By doing away with border checks and reducing bureaucracy, it provides a powerful economic boost and enhances the notion of a common European identity, they say.
Berlin's decision Wednesday to extend greater passport controls to its border with France -- Schengen's other principal architect -- seemed to deal the accords another blow.
German Chancellor Angela Merkel and her Austrian counterpart Werner Faymann on Tuesday called for a special EU summit next week to debate the crisis.
"Time is running out," Merkel warned, urging an end to the squabbling that has flared since eastern members flatly refused to accept EU-set quotas for taking in tens of thousands of refugees.
European President Donald Tusk has said he will announce a decision about the summit on Thursday, with EU interior ministers to meet again next Tuesday in a fresh bid to resolve the quota wrangle.