Macedonia has stepped up security on its border with Greece, stranding thousands of migrants in a dusty field.
The government proclaimed a state of emergency and said it is deploying troops as it tries to stem a surge of migrants from the Middle East, Asia and Africa.
Police spokesman Ivo Kotevski said police and the army will control a 30-mile stretch of the border to stop a "massive" influx of migrants coming from Greece.
"This measure is being introduced for the security of (Macedonian) citizens who live in the border areas and better treatment of the migrants," he said.
The measure could create a huge backlog of migrants on the Greek side of the border, from where 2,000 a day have been illegally crossing into Macedonia, an impoverished country overwhelmed by the surge.
Hundreds of migrants travelling from the Greek island of Kos reportedly plan to move into Macedonia in the next few days.
Until now, the border has been porous, with only a few patrols on each side. Sealing it would disrupt the so-called Balkan corridor for migrants who start in Turkey and take boats to Greece or walk to Bulgaria, then make their way through Macedonia or Serbia en route to the wealthier countries of the European Union.
Almost 39,000 migrants, most of them Syrians, have been registered passing through Macedonia over the past month, double the number from the month before.
Thousands of migrants are stranded in a no-man's land near the Macedonian town of Gevgelija, from where they plan to catch trains to the Serbian border on their way to Hungary, which is building a fence to try to keep them out. A police helicopter hovered nearby and officers in armoured vehicles watched the crowd.
Migrant Ahmet Husa, from Syria, said: "People from Syria escaped from war, escaped death and we want to see our future in Europe. We need this road to see our future."
For months, the train station in Gevgelija was the scene of skirmishes between baton-wielding policemen and migrants trying to get on overcrowded trains.