Hungarian riot police have fired tear gas and water cannon to force back migrants from the Serbia-Hungary border.
Hundreds of people have massed at a closed crossing point near the Serbian town of Horgos, and are involved in a tense stand-off with police on the other side of the border.
Some migrants threw missiles, including stones and water bottles.
Many of the migrants want to reach Germany via Hungary.
The crossing point has been used by tens of thousands of migrants to enter Europe's Schengen zone, which allows people to travel between member countries without restrictions.
Hungary closed its border on Tuesday after enacting new laws which make it illegal to enter the country or damage a new razor-wire border fence, and the country's courts have started fast-track trials of arrested migrants.
Television pictures showed fires burning, and police vehicles and ambulances arriving on the Serbian side of the border, across from the massed ranks of Hungarian riot police on the other side.
The AP news agency reports that several people received medical treatment from the Serbian ambulance service at the scene of the clashes - most were suffering the effects of tear gas but one young man had a bloody leg.
"We fled wars and violence and did not expect such brutality and inhumane treatment in Europe,'' the agency quoted Amir Hassan, from Iraq, as saying.
The Hungarian foreign minister as saying he has asked Serbia to act against migrants who "attacked" police on the border.
Meanwhile, Croatia has said it will allow migrants to travel on to northern Europe, opening up a new route a day after Hungary sealed its border with Serbia.
More than 150 migrants have crossed into Croatia from Serbia, with some of those stranded on Serbia's border with Hungary now using the same route.
New border restrictions and a row over allocating migrants have exposed bitter divisions in Europe over the crisis.