New York, US (BBN)-UN Secretary General Ban Ki-moon has said he is "shocked" after Hungarian police fired tear gas and water cannon to force migrants back from its border.
Ban said such treatment of asylum seekers was "unacceptable", reports BBC.
Hundreds of migrants were involved in the clashes at the border between Hungary and Serbia on Wednesday, trying to break through a razor-wire fence.
Many of them want to reach Germany, amid divisions within the European Union over how to deal with the crisis.
Hungary defended its action, saying that 20 police officers were injured as migrants tried to break through a gate, and a spokesman accused migrants of using children as "human shields".
At least two migrants were also injured, Hungarian and Serbian officials said.
Hungary closed its entire border with Serbia on Tuesday after making it illegal to enter the country or damage the border fence.
The country's courts have started fast-track trials of arrested migrants.
More than 200,000 people have already crossed into Hungary this year to enter the EU's Schengen zone, which normally allows people to travel between member countries without restrictions.
IN OTHER DEVELOPMENTS:
EU Migration Commissioner Dimitris Avramopoulos is due to visit refugee centres in Bicske, Hungary, and Rosenheim, Germany
Top Austrian, Croatian and Slovenian officials will hold talks to discuss in detail the passage of migrants into Austria
A steady stream of migrants has already been crossing into Croatia from Serbia
One of the main motivations driving Syrians towards Europe is to find a better life and education for their children, the Save the Children says. Its report says more than two million children inside Syria are unable to attend schools
'BRUTAL' TREATMENT'
On Wednesday, there were chaotic scenes near the Hungarian town of Horgos, with fires burning and police vehicles and ambulances arriving on the Serbian side of the border, across from massed ranks of riot police on the other side.
Some migrants threw missiles, including stones and water bottles.
The firing of tear gas and water cannon created a stampede of migrants away from the border.
Several people received treatment from the Serbian ambulance service, some suffering the effects of tear gas.
Migrant Amir Hassan, from Iraq, said: "We fled wars and violence and did not expect such brutality and inhumane treatment in Europe."
Serbian Prime Minister Aleksandar Vucic accused Hungary of being "brutal and "non-European".
Hundreds of people who braved batons and barbed wire at Serbia's Hungarian crossing are now trekking down a dirt track through dried cornfields en route to Croatia.
The contrast is stark, if not surreal.
Croatian border guards sporting baseball caps greet them in English: "Don't be afraid."
"Where are we now?" some ask. Many pause at the sight of police vans.
UN and Red Cross officials reassure them they're only being taken to buses to ferry them to a centre in Zagreb for some rest and registration.
So many are arriving, a special train service has also started.
UN officials told me that by late Wednesday evening that more than 1,200 had crossed and the flow was continuing, even through pitch black fields, as packed buses and taxis continued to turn up on the Serbian side.
"It's sinking in that Hungary is not going to let them in," said UNHCR's Melica Sunjic.
But, further north, Slovenia's prime minister announced he was tightening his borders "until we find a common European solution."
So far, there's no sign of that.
Serbia has said it will send additional police to its border with Hungary.
The UN High Commissioner for Refugees, Antonio Guterres, called on the Hungarian authorities to ensure "unimpeded access" for people fleeing wars and persecution.
"UNHCR was especially shocked and saddened to witness Syrian refugees, including families with children who have already suffered so much, being prevented from entering the EU with water cannon and tear gas," a statement said.
BBN/SK/AD