By Ivana Sekularac and Krisztina Than

SERBIAN-HUNGARIAN BORDER (Reuters) - Hungary sealed off a railway track on Monday used by tens of thousands of migrants to enter the European Union on foot, launching a crackdown promised by the right-wing government to tackle Europe’s worst refugee crisis in two decades.

With tough new laws and powers to turn back asylum seekers entering into force at midnight, helmeted police officers, some on horses, took up position across the track that traverses the border and has been used for months by migrants, many of them Syrian refugees, to enter the EU from Serbia.

A helicopter circled overhead and a cargo wagon, one end covered in razor-wire, was shunted into position to seal the border.

The scene marked the first step in what Prime Minister Viktor Orban, one of Europe’s most vociferous opponents of immigration, says is a new policy of zero tolerance on the EU’s external border.

As night fell, migrants continued to cross, but in small groups queuing at an official pedestrian border crossing around a kilometer away. Workers from the United Nations refugee agency, UNHCR, brought blankets to protect them against the late summer cold.

“It is difficult to say what will happen after midnight. No one can say what will happen,” said UNHCR regional spokeswoman Melita Sunjuc.

A major flashpoint in Europe’s worst refugee crisis since the Yugoslav wars of the 1990s, Hungary has vowed to get to grips with the flow through the Balkan peninsula to the richer countries of northern and western Europe.

Police have recorded over 190,000 entering Hungary this year, many of them fleeing conflict in the Middle East and Africa, including a record for one day of 7,437 by 4 p.m. (10 a.m. EDT) on Monday.

Their window of opportunity is rapidly shrinking as Hungary races to finish a 3.5-metre high fence the length of its 175-km (108 mile) southern border by October.

As of Tuesday, those who enter illegally over or under the fence risk arrest and imprisonment. Those who seek asylum face being held while their claims are fast-tracked, and possibly expelled back into Serbia after Hungary in July declared its impoverished neighbor a ‘safe country’ for refugees. Rights groups and the UNHCR dispute the designation.

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Orban said he expected a high rate of deportations.

“In such a case, if someone is a refugee, we will ask them whether they have submitted an asylum request in Serbia,” Orban was quoted as telling private broadcaster TV2. “If they had not done so, given that Serbia is a safe country, they will be rejected.”

The influx into Europe, by boat from North Africa across the Mediterranean or across Turkey and up the Balkan peninsula, has triggered discord and recrimination in the 28-nation EU, feeding anti-immigration sentiment.

On Monday, two decades of frontier-free travel across Europe unraveled as Austria and Slovakia followed Germany in re-establishing border controls to cope with the influx. Austria said it would dispatch armed forces to guard its eastern frontier with Hungary.

"UNCOMPROMISING"

Soldiers were already on the Hungarian border with Serbia on Monday and Orban drafted in hundreds more police, telling them to be humane but “uncompromising” in implementing the new laws.

“You will meet with people who have been deceived. You will be met with temper and aggression,” he told them.

Hungary’s National Transport Authority said it had closed Hungarian airspace in a 20-km band along the Serbian border up to a height of 1,350 meters to ensure the police, army and ambulance services could act effectively. International passenger flights were not affected.

The prospect of harsher measures and a long wait in Hungary as of Tuesday appeared to have spurred many migrants, who rushed by train, bus and taxi north through the Balkans having landed in Greece by boat and dinghy from Turkey.

“We heard the Hungarians will close the border on September 15th so we had to hurry from Greece,” 24-year-old engineering student Amer Abudalabi, from the Syrian capital Damascus, said shortly before crossing the border from Serbia.

“We have not slept since Saturday morning… I’m so tired. I won’t believe it when we cross into Hungary.”

Before shutting down the informal border crossing, police shepherded several thousand migrants to trains bound direct for the Austrian border, in an apparent attempt to clear the backlog by Tuesday.

Some said they had not been registered, despite the government’s insistence it is sticking to EU rules that asylum seekers must be registered in the first EU country they enter.

The UNHCR said there was “no official procedure, people are just being collected”.

A government spokesman denied authorities were no longer registering migrants, saying registration was taking place elsewhere.

Hours later, directed to the official border crossing, 25-year-old Syrian civil engineering student Mohamed stood in a long line waiting to enter Hungary.

“We don't know what will happen to us,” he said. “I want to go to Germany, but they will first take our fingerprints and then they will tell us where to go.”

(Additional reporting by Sandor Peto and Gergely Szakacs in Budapest and Maja Zuvela in Belgrade, Writing by Matt Robinson; editing by Anna Willard)