"I believe it’s not in anyone’s interest to violate the principles of the Schengen area based on the free movement of citizens," minister Robert Kalinak wrote to his counterpart Maria Theresia Fekter, demanding an explanation.

The Austrian police on Saturday stopped several busses with Slovaks aboard on a newly-launched line between the Slovak capital Bratislava and the nearby Austrian border town of Hainburg.

"The police were checking the passengers’ ID cards," a spokesman for the bus line operator DPB Peter Kavecky told AFP.

"The Slovak citizens don’t need a passport to travel to Austria since the country joined the Schengen zone in December 2007, but people with children, who didn’t have any IDs, had to get off the bus and walk back to the Slovak side of border," he said.

The police controls haven’t been repeated since Saturday, he added.

Czech authorities have raised similar protests in the past against German police checking Czech tourists travelling by car or by bus.

AFP/Expatica