ATHENS: Thousands of migrants were stranded in northern Greece on Monday after neighboring Macedonia demanded additional identification from people seeking to cross the border and head to Western Europe, witnesses said.

European leaders are concerned that migrants passing through austerity-hit Greece to more prosperous countries could end up stranded if Greece’s northern neighbors tighten border controls.

Greek officials say the flow of people across the border slowed after Macedonia demanded additional identification from people seeking passage.

About 5,000 people massed at two locations in northern Greece, close to the border with Macedonia, while aid groups urged another 4,000, who arrived on the Greek mainland from outlying islands, not to head to north for fear of creating a bottleneck.

Separately, the German government Monday condemned the “cold-hearted and cowardly” mob that tried to stop a bus from taking migrants to a shelter for asylum seekers, calling the ugly episode “deeply shameful.”

About 100 people in the Saxony town of Clausnitz shouted “We are the people” and tried to block the bus carrying about 20 asylum seekers on Thursday night.

The images captured on video, which show some terrified migrants crying, have gone viral on social media, sparking widespread outrage.