SERBIAN-HUNGARIAN BORDER (REUTERS) - Crowds of migrants built up at Serbia's northern border with Hungary on Tuesday (Sept 15), their passage into the European Union blocked by a razor wire fence they can now no longer struggle through or go around.

A day after the pressure of people seeking refuge from war and poverty tore up two decades of frontier-free travel within Europe, Hungary effectively shut this entrance to the EU in scenes carrying echoes of the Cold War.

Families with small children sat in fields beneath the former communist country's new 3.5-metre high fence running almost the length of the EU's external border with Serbia, halted by a right-wing government that hailed a "new era".

Rules which came into force at midnight will send them back if they seek asylum and mean arrest if they breach the fence, which thousands did while it was being built.

Long queues formed in no-man's land at metal containers built into the fence, where migrants were expected to register, though only a handful were seen entering. They had spent the night in the open, given tents, food and water by aid workers.

Nine Syrians and seven Afghans were detained by police and face possible imprisonment on suspicion of breaching the fence, the first arrests under the new rules.

Hungary says it will let refugees request asylum. But under the new regime, they risk expulsion within eight days to Serbia, which Budapest has declared 'safe' for refugees. "I don't know what I will do," said 40-year-old Riad from Aleppo, once Syria's commercial hub reduced in many parts to rubble since war broke out in 2011 and put to flight millions of Syrians. "I will wait to see. We have lost everything to reach this point."

The influx of hundreds of thousands of migrants and refugees from the Middle East, Africa and Asia has triggered discord and recrimination in Europe.

EU ministers failed to break a deadlock on Monday over sharing out responsibility for some of those seeking asylum. Austria and Slovakia followed Germany in re-establishing border controls and Austria said it would dispatch armed forces to guard its eastern frontier with Hungary.

At least 200,000 migrants have crossed Hungary so far this year, streaming north through the Balkan peninsula having hit Greek shores by boat and dinghy from Turkey. More than 9,000 entered on Monday, a record for the year.

Hungarian Prime Minister Viktor Orban, one of Europe's most vociferous opponents of immigration, has vowed to stop the flow.

The prospect of a long wait at the Hungarian border, possible imprisonment or expulsion back to Serbia may force many to seek alternative routes.

They could go west into Serbia's fellow former Yugoslav republic Croatia, or east into Romania, both members of the EU like Hungary but not of Europe's Schengen zone of border-free travel.

Others may bide their time at the fence, where the razor wire and soldiers resembled the borders of eastern Europe during the Communist era.

Serbia, an impoverished ex-Yugoslav republic years away from joining the EU, fears becoming the latest bottleneck of the migrant crisis. The government says it is readying more temporary accommodation, but warned it would not accept anyone turned back from Hungarian territory. "That's no longer our responsibility," Aleksandar Vulin, the minister in charge of policy on migrants, told the Tanjug state news agency. "They are on Hungarian territory and I expect the Hungarian state to behave accordingly towards them." The United Nations refugee agency, UNHCR, says it disputes the designation of Serbia as a so-called "safe third country", which would imply refugees have a fair chance of being granted asylum and would receive the necessary protections and support.

Rights groups say Serbia meets none of the criteria and is still finding homes for thousands of its own refugees from the collapse of Yugoslavia in the 1990s, the last time Europe confronted displacement of people on such a scale.

Orban said he expected a high rate of deportations, framing the crisis as a battle for Europe's prosperity, identity and"Christian values". "In such a case, if someone is a refugee, we will ask them whether they have submitted an asylum request in Serbia. If they had not done so, given that Serbia is a safe country, they will be rejected," he was quoted as telling private broadcaster TV2 on Monday.

An official of Orban's Fidesz party said authorities would rule on such asylum requests within eight days. "We will start a new era," government spokesman Zoltan Kovacs said shortly after midnight on the border. "We will stop the inflow of illegal migrants over our green borders." The migrants' window of opportunity is rapidly shrinking as Hungary puts the finishing touches to the fence the length of its 175-km (108 mile) southern border.

On Monday, a cargo wagon, one end covered in razor wire, was shunted into position to close the main informal crossing point for migrants. Helmeted police and soldiers stood guard and a helicopter circled overhead. "We're on the street now," said Mouz, a 22-year-old Syrian, who slept on the border. Asked if he might consider another route, he replied: "I don't know. I'm from Syria. I cannot go back."