Record numbers of refugees are streaming into EU member Hungary from Serbia, posing a new headache for regional leaders at a summit this week set to be dominated by Europe's worst migrant crisis since World War II.

Almost 2,100 people, the highest ever daily total, crossed the border near the Hungarian town of Roszke on Monday, police said, just days before Hungary completes a vast razor-wire barrier to keep out migrants.

They were part of around 7,000 refugees whose hazardous journey to the European Union had been temporarily blocked last week when Macedonia declared a state of emergency and shut its borders for three days to halt the huge influx of people mostly fleeing war in Syria.

Authorities reopened the crossing after chaotic scenes involving police lobbing stun greandes at the migrants trying to break through the border.

"We were stopped in Macedonia for two days, the riots were terrible, police used guns and teargas, I saw an old woman beaten, her money and papers taken," said a 29-year-old IT engineer from Mosul in Iraq who said he had left his home to escape the Islamic State group. He asked not to be named.

The UN's refugee agency said on Tuesday it expected the number of refugees moving through Macedonia to double from around 1,500 per day to 3,000 per day, many of them women and children.

It warned that the situation was also worsening on the shores of Greece and Italy, where the number of Mediterranean sea crossings was now approaching 300,000.

Since the beginning of the year, more than 2,370 have drowned in the Mediterranean, already exceeding the death toll for the whole of 2014, according to the International Organization for Migration. (AFP)