The action followed clashes between demonstrators and police on U.S. campuses and a blockade at Paris's Sciences Po university
"We estimate that around 4,000 have arrived - and I don't think that is the end of it," said Helmut Marban, spokesman for the police in the province of Burgenland, adding some already had continued their journey onwards.
"The numbers could well double, if not more," he told Reuters. A special half-hourly train service was now running to take people from the border to Vienna on top of 20 buses shuttling back and forth between the Austrian capital and the border with Hungary.