Austria has partially shut a motorway linking Vienna and Hungary as dozens of migrants who had crossed the border set off for the Austrian capital on foot rather than wait for struggling authorities to arrange transport.
Austria closed the A4 motorway near the Hungarian border for security reasons on Friday morning.
Around 8,000 people crossed the border into Austria on Thursday and a further 3,600 had crossed since midnight, a police spokesman said.
Austria cut its rail link to Hungary on Thursday, citing a "massive overburdening" of its capacity by migrants who continue to stream in from Hungary, many of them having fled the civil war in Syria and marched through the western Balkans. The train line will remain closed over the weekend, the rail company said.
Since Germany and Austria said last week that they would let the wave of migrants enter their territory, tens of thousands of people have passed through Austria on their way to Germany. Only hundreds have requested asylum in Austria.
One witness saw hundreds of people crossing the Hungarian border into Austria on Friday morning.
"Around 1,500 people have spent the night outdoors," a police spokesman said. Authorities were operating 30 buses and were trying to move people "to where they can continue their journey", he added.
The police handling the new arrivals at the Hungarian border are struggling to provide food and shelter for the migrants while arranging transport - largely on buses, since the rail link to Vienna is suspended.
Hungary has transported tens of thousands of people to near the Austrian border in the past week. They have then crossed the border on foot. Others have entered Austria by train, but that option has been unavailable since Thursday's rail suspension.
The Austrian government held a special meeting yesterday to discuss the migrant crisis.
The 'Wiener Zeitung', a daily newspaper, said the crisis would cost Austria an extra €1bn.
Meanwhile, the 71 badly decomposed bodies found in the back of an abandoned truck in Austria last month have been released for burial.
The discovery of the remains, presumed to be those of refugees who had to rely on people smugglers to help them cross the border between Austria and Hungary, caused widespread shock and drew attention to the conditions in which many have fled conflict and poverty in the Middle East, Africa and elsewhere.
The van was discovered on the same motorway that was shut down yesterday.
Days after they were found, Austria and Germany threw open their borders to tens of thousands of people making their way north across Europe via Greece and the Balkans.
"The result of the Vienna pathologists' post-mortem report is clear, although special investigative methods could only be used in a limited way because of the bodies' advanced state of decomposition," an official statement said.
The bodies included those of four children.
A spokeswoman for the prosecutor's office in the eastern town of Eisenstadt, which is handling the investigation into the deaths, said that the people were presumed to have suffocated in the back of the truck.