ROSZKE: Two decades of frontier-free travel across Europe unravelled on Monday as countries re-established border controls in the face of an unprecedented influx of migrants, which broke the record for the most arrivals by land in a single day.

Germany's surprise decision to restore border controls on Sunday had a swift domino effect, prompting neighbours to impose checks at their own frontiers as thousands of refugees pressed north and west across the continent while Hungary sealed the main informal border crossing point into the European Union.

A majority of EU interior ministers, meeting in Brussels, agreed in principle to share out 120,000 asylum seekers on top of some 40,000 distributed on a voluntary basis so far, EU president Luxembourg said. But details of the deal, to be formalised on Oct. 8, were vague with several ex-Communist central European states still rejecting mandatory quotas.

Austria said it would dispatch its military to help police carry out checks at the border with Hungary after thousands of migrants crossed on foot overnight, filling up emergency accommodation nearby, including tents at the frontier.

Thousands more raced across the Balkans to enter Hungary before new rules take effect on Tuesday, which Budapest's right-wing government says will bring a halt to the illegal flow of migrants across its territory.

By 1400 GMT on Monday, police said 7,437 migrants had been recorded entering Hungary from Serbia, beating the previous day's record of 5,809.

Then helmeted Hungarian police, some on horseback, closed off the main informal crossing point, backed by soldiers as a helicopter circled overhead. A goods wagon covered with razor wire was moved into place to block a railway track used by migrants to enter the EU's Schengen zone of border-free travel.

Hungary later declared the low-level airspace over its border fence closed but allowed a trickle of refugees to enter the country at an official crossing point.

As the shockwaves rippled across Europe, Slovakia said it would impose controls on its borders with Hungary and Austria. The Netherlands announced it would make spot checks at its borders. Other EU states from Sweden to Poland said they were monitoring the situation to decide whether controls were needed.

"If Germany carries out border controls, Austria must put strengthened border controls in place," Vice Chancellor Reinhold Mitterlehner told a joint news conference with Chancellor Werner Faymann. "We are doing that now."

The army would be deployed in a supporting role.