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.
Mr Ban said such treatment of asylum seekers was "unacceptable".
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.
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".
Source: BBC News
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