Croatia became Thursday the latest hotspot in Europe's migrant crisis, with thousands of new arrivals bound for northern Europe overwhelming local authorities as the EU's president called an emergency summit next week.
Meanwhile, in a sign of growing pressure on the EU's other external borders, Bulgaria began deploying 1,000 troops to the Turkish frontier where several hundred people, mostly Syrians, spent a third day stuck near the border city Edirne.
After Hungary this week sealed off its border with Serbia, cutting off one of the main routes into the EU for hundreds of thousands of migrants this year, 7,300 had entered Croatia by midday (1000 GMT) at four crossings, with authorities bracing for 20,000 in the next two weeks.
A day earlier, Hungarian riot police fired tear gas and water cannon in several hours of clashes with rampaging migrants angry at being blocked from entering the country and throwing stones, sticks and plastic bottles.
Croatia initially welcomed the migrants but as it emerged that the country appeared to have replaced Hungary at the end of the Balkans route into the EU, Prime Minister Zoran Milanovic warned that there were "limits to our capacities".
A special train transporting 800 arrived near Zagreb from the border early Thursday, but thousands more were waiting in vain for trains in baking sun at a tiny station near the Serbian frontier, an AFP correspondent said.
With the Red Cross running out of food and water and local official Bozo Galic warning of a "humanitarian disaster", early afternoon buses finally began to arrive to take people to another station at Ilaca to board a train to Zagreb.
But there were more arriving than could be taken away and police were clearly losing control of the situation, with several hundred people breaking through a barrier outside the station and starting to walk away from the town.
"They need to do something fast. Thousands more are coming from Serbia. In one or two hours there will be a revolution," said one Iraqi as a young boy was passed over the barrier to a Red Cross worker, distraught at having lost his parents.
"We are happy to pay. Why are they keeping us here?" said Syrian Abdullah Janabi, 22.
The first train from Ilaca with around 800 migrants on board left late afternoon Thursday.
Hungary, which until sealing its border with Serbia had seen more than 200,000 migrants enter this year, angrily rejected a growing chorus of criticism of its handling of Wednesday's clashes which left 14 police injured.
- Hungary rejects criticism -
"The police acted necessarily and proportionately, huge credit to them," said government spokesman Zoltan Kovacs. "The weeks since the building of the border closure show that Hungary alone amongst member states is capable of keeping the Schengen rules."
UN Secretary-General Ban Ki-moon on Wednesday said he was "shocked" by Hungary's actions, saying people "fleeing war and persecution... must be treated with human dignity."
Serbian Prime Minister Aleksandar Vucic condemned the "brutal behaviour" of Hungary's police, saying he would not "allow anybody to humiliate us and fire tear gas on Serbia's territory."
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- Political pressure -
"The use of violence, patrols with guns and pushing innocent war victims into Balkan minefields is behaviour unbecoming of an EU member state," Greek foreign ministry spokesman Constantinos Koutras said.
The situation at the flashpoint Roszke crossing was calm on Thursday, with around 300 migrants left after around 10 buses came and took people away to the Croatian border.
Hungarian police on the other side had also reduced in number.
Pressures mounted elsewhere meanwhile.
In Turkey, some 500 migrants spent another night in a makeshift camp on the main road into Edirne, sealed off by police to refugees on Tuesday. Another 500 slept at Istanbul bus station.
The refugees see the route through Edirne as offering a better chance of survival than boarding an overcrowded boat to a Greek island.
"There is no return to Istanbul, there is no return to the sea, there is no return to Syria," Ahmed, a 25-year-old Syrian translator told AFP.
In Paris, authorities Thursday started clearing out two makeshift camps housing more than 500 migrants and offered them accommodation elsewhere.
And Germany reported a sharp rise in migrant arrivals on Wednesday to 9,100.
Almost all crossed from Austria, with 2,000 by train and the rest by road or on foot, three days after Germany -- which expects 800,000 to a million arrivals this year -- introduced passport checks on borders.
In Brussels, EU president Donald Tusk said that all 28 leaders of the bloc would next Wednesday hold an emergency summit on the continent's worst migration crisis since World War II.
- Crisis EU summit -
The bloc is bitterly split and free movement across borders -- a pillar of the European project -- is in jeopardy.
One of the big fears is for the future of Europe's 20-year-old Schengen agreement, which allows borderless travel between some member states, and is considered as important as the euro by many EU supporters.
Germany, Austria and Slovakia have all reimposed identity checks on parts of their borders, and Poland and the Netherlands are considering whether to follow suit.