Greek police officers direct buses carrying hundreds of migrants, who were stranded on the Greek-Macedonian border and blocked rail traffic, after a police operation near the village of Idomeni, Greece, December 9, 2015.

Greek police launched an operation early Wednesday to remove a thousand migrants gathered at the border with Macedonia, after Skopje introduced a system of filtering those who can cross the frontier by nationality.

Some 400 riot officers were mobilized at dawn, a police source said.

Humanitarian workers in the area had been asked to leave while reporters and photographers, some of whom were briefly detained, were held back at a distance of about three kilometers (two miles).

The migrants in questions, mostly Moroccans and Pakistanis, were led away to buses, ten of which left for Athens in the morning, according to the police source.

The departures were carried out in a calm atmosphere, she added.

In the capital they will be hosted at migrant centers. Those who want can file for asylum while the others will be repatriated.

Meanwhile police began to dismantle migrant tents which had been blocking a rail line for the past month, leading to losses for the rail operator and Greek exporters.

With Macedonian authorities only letting through refugees from Syria, Iraq and Afghanistan, migrants from other countries have been blocked on the border with Greece for days or even weeks.

Their anger boiled over last week when groups of migrants seized communal tents operated by humanitarian agencies and destroyed some prefab houses set up by the UN refugee agency.

One migrant died of electrocution after clashes broke out.

After previously excluding the use of force to clear the area, in favor of a bid to persuade the migrants to leave voluntarily, Migration Minister Iannis Mouzalas, decided that intervention was necessary as European pressure and local complaints grew.

Greece is a staging post on the route towards the Balkans and on to western Europe for hundreds of millions of migrants arriving from Turkey.

The Greece-Macedonia border at Idomeni railway station, just across from Gevgelija on the Greek side, was closed on November 18 by the Macedonian authorities, with only those arriving from conflict zones — Syria, Iraq and Afghanistan — allow to pass.

This heightened tensions between those deemed bona fide refugees and those stuck at the border with no prospect of being allowed through.

Next week member’s of the European Union’s Frontex border agency are due to deploy in the area.

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