Twenty-year travel to Europe without borders has been suspended on Monday after numerous countries have established surveillance at their borders in response to the unprecedented inflow of migrants.

The sudden Sunday decision by Germany to return border surveillance led to a domino effect, as it also forced neighboring countries to close their own borders. Austria sent an army to the border with Hungary after thousands of migrants went on foot overnight.

"If Germany conducts border control, Austria has to set up more controllable border points, and that's what we are doing," Vice-Chancellor Reinhold Mitterlehner told the press conference, adding that the army would be deployed as a support.

"Support will primarily focus on humanitarian aid," said Chancellor Werner Faymann, who was at the same press conference. "But I want to emphasize that there will be support in border control if that is necessary."

And Slovakia announced the closing of the border to Germany and Austria.

These measures have so far been the biggest threat to the Schengen system, which is 1995. abolished border crossings across Europe and which, together with the common currency of the euro, is considered a transformative achievement of the continent's integration.

The current rules defy immigrants without personal documents traveling within the Schengen area, but lack the mechanisms that would actually prevent them.

Poland also announced the readiness to set border control in the case of a large number of migrants, and any EU decision to impose a quota for refugee acceptance by individual Member States, advocated by the European Commission and some of the larger German-led member countries, would lead to an institutional crisis.

"We will only accept as many refugees as we can afford, no more or less," Prime Minister Ewa Kopacz said.

Members of the Schengen area have the right to re-establish temporary border control in crisis situations, and occasionally they did it in the past, but never at the level at which this happened on Monday.