Refugees, migrants push past police in Croatia on their way to Western Europe
Thousands of refugees and migrants have poured into Croatia from Serbia in a rush to get on limited buses and trains.
Dozens were injured in the melee as some guards stood behind trees to protect themselves.
Croatian police said 7,300 people had entered since the first groups started arriving on Wednesday.
The Balkan nation is the latest hotspot in the 1,000-mile plus exodus towards Western Europe after Hungary sealed off its border on Tuesday with a razor-wire fence before using tear gas, batons and water cannons to keep people out.
Croatia represents a longer and more arduous route into Europe but those fleeing violence in their homelands have little choice.
After bus trips through Serbia, many migrants crossed fields on foot to enter Croatia, where dozens of police directed them to trains and buses heading to refugee centres. Authorities warned them to avoid walking in areas along the Serbian border that are still being de-mined from the country's 1991-95 war.
Hundreds of asylum seekers pushed through police lines in the eastern Croatian town of Tovarnik after waiting for hours in the hot sun.
More than 2,000 men, women and children had been stuck at the local train station for hours. When buses finally arrived, groups charged toward them, overwhelming Croatian police.
Hundreds of other asylum seekers came over a Danube River bridge to the northern Croatian town of Batina after being transported there by Serbs, overwhelming the local police.
The migrants are unlikely to stay long in Croatia, where they must be registered. They can ask for asylum in Croatia, but almost all plan to travel on, passing through Slovenia and then Austria en route to Germany or the Scandinavian countries.
Fearing a surge of migrants from Croatia, Austria and Slovenia have called for an urgent, all-EU response. Both have reinstated border checks.
"We are being put to test," said Austrian Chancellor Werner Faymann. "This time we must prove that we don't want a Europe in which everyone will try to shift their problems to others' shoulders."
Hungary says it plans to build more fences along its borders, this time with Romania and Croatia.
Romania has expressed concern after Hungary said the fence would be more than 43 miles long.
Clashes between migrants and Hungarian riot police broke out on Wednesday afternoon after people frustrated at being blocked from the country pushed open a gate at the Horgos border crossing with Serbia. Dozens were injured as police responded with tear gas, pepper spray and water cannons, and migrants threw rocks and other objects at them.
Hungarian foreign minister Peter Szijjarto said: "I find it bizarre and shocking that certain esteemed international figures have stood on the side of people who for hours were throwing stones and pieces of cement at the Hungarian police.
"And I'd also like to make it very clear, no matter what criticism I receive, that we will never allow such aggressive people to enter Hungary. Not even for transit purposes."
Hungarian police said they detained 22 people, including one Syrian man who they said was an organiser at the Horgos clash who is now suspected of terrorism.
The European Union's migration commissioner, Dimitris Avramopoulos, declared that walls and violence are no solution and urged Hungary to work with the 28-nation bloc to alleviate the continent's migration crisis.
"The majority of people arriving in Europe are Syrians," Mr Avramopoulos said at a news conference alongside Szijjarto in Budapest. "They are people in genuine need of our protection. There is no wall you would not climb, no sea you wouldn't cross if you are fleeing violence and terror. I believe we have a moral duty to offer them protection."
Hungary, in contrast, has been insisting that most are simply economic migrants seeking better jobs. Prime Minister Viktor Orban has also said that by keeping out Muslims, Hungarian police are defending "Europe's Christian culture".
Mr Avramopoulos said he considered it a "Christian duty" to handle the migration crisis with compassion.
In Paris, meanwhile, French authorities evacuated more than 500 Syrian and other migrants from tent camps and moved them into special housing.
And at Turkey's border with Greece and Bulgaria, 200 asylum seekers, many from Syria, protested in the north-western province of Edirne, demanding to be let into Europe.
Croatian Prime Minister Zoran Milanovic said his country cannot hold migrants who wish to move on toward Western Europe, claiming "our resources are limited".
He added: "We will not and cannot keep them in Croatia and no one will make us do that."
Later Slovenian police said a group of about 150 refugees who have arrived from Croatia on an international train bound for Zurich will be returned to Croatia.
The refugees arrived at the Dobova train station, which serves as the main rail border crossing between Slovenia and Croatia. They were transferred to three cars which will take them back to Croatia, according to Slovenia's state news agency.
Meanwhile, Croatia's Interior Minister says his country could close down its borders if faced with thousands more migrants and refugees coming in from Serbia.
Ranko Ostojic urged Serbia's authorities to halt the migrants.