THESSALONIKI – Hundreds of migrants are gathering outside a camp near the northern Greek city of Thessaloniki, planning a long trek through heavily guarded Balkan borders to seek asylum in Europe’s prosperous heartland.

Police said more than 500 people, including families with small children, gathered Thursday in a cornfield outside the Diavata migrant camp, some 10 kilometers (6 miles) west of Thessaloniki, some setting up tents. Dozens more were approaching on foot.

Migrants said they were responding to an anonymous call on social media to go to Greece’s border with North Macedonia 60 kilometers (38 miles) away and push through, reviving a migration route that was firmly shut down in 2016.

The United Nations refugee agency has warned migrants against the social media calls, saying irregular border crossings are “risky and dangerous.”

A migrant holds a baby during a rally outside refugee camp in the village of Diavata, west of Thessaloniki, northern Greece, Thursday, April 4, 2019. About 300 migrants from various parts of Greece gathered outside the Diavata camp, in response to an anonymous posting on social media that urged migrants trapped in Greece to go to the border with North Macedonia and try to push their way through _ with a view to crossing a series of sealed Balkan borders and reaching Europe’s prosperous heartland. (AP Photo/Giannis Papanikos)

Protesting migrants gather as riot police guard a refugee camp in the village of Diavata, west of Thessaloniki, northern Greece, Thursday, April 4, 2019. About 300 migrants from various parts of Greece gathered outside the Diavata camp, in response to an anonymous posting on social media that urged migrants trapped in Greece to go to the border with North Macedonia and try to push their way through _ with a view to crossing a series of sealed Balkan borders and reaching Europe’s prosperous heartland. (AP Photo/Giannis Papanikos)

A boy stands over a baby during a rally outside a refugee camp in the village of Diavata, west of Thessaloniki, northern Greece, Thursday, April 4, 2019. About 300 migrants from various parts of Greece gathered outside the Diavata camp, in response to an anonymous posting on social media that urged migrants trapped in Greece to go to the border with North Macedonia and try to push their way through _ with a view to crossing a series of sealed Balkan borders and reaching Europe’s prosperous heartland. (AP Photo/Giannis Papanikos)

(AP Photo/Giannis Papanikos)

A migrant holds a baby during a rally outside refugee camp in the village of Diavata, west of Thessaloniki, northern Greece, Thursday, April 4, 2019. About 300 migrants from various parts of Greece gathered outside the Diavata camp, in response to an anonymous posting on social media that urged migrants trapped in Greece to go to the border with North Macedonia and try to push their way through _ with a view to crossing a series of sealed Balkan borders and reaching Europe’s prosperous heartland. (AP Photo/Giannis Papanikos)

The migrant camp in Diavata. (AP Photo/Giannis Papanikos)

FILE – Migrants gather during a rally outside a refugee camp in the village of Diavata, west of Thessaloniki, northern Greece, Thursday, April 4, 2019. (AP Photo/Giannis Papanikos)