Greek authorities began a dawn operation to evacuate the country's largest informal refugee camp of Idomeni, blocking access to the area and sending in more than 400 riot police.

The government's spokesman for the refugee crisis, Giorgos Kyritsis, said on Monday that the operation on the Macedonian border was expected to last about a week to 10 days.

The camp, which sprung up on what began as an informal pedestrian border crossing for refugees and migrants heading north to Europe, is home to an estimated 8,400 people.

Greek police and government authorities have said the residents will be moved gradually to newly completed, organised camps.

Journalists were barred from the camp, stopped at a police roadblock a miles away on a highway junction leading to the nearby village of Idomeni. Twenty buses carrying various riot police units were seen heading to the area while a police helicopter observed from above.

More than 54,000 refugees and migrants have been trapped in financially struggling Greece since Balkan and European countries shut their land borders to a massive flow of people escaping war and poverty at home.

The vast majority are from Syria, Iraq and Afghanistan. Nearly a million people have passed through Greece, most arriving on islands from the nearby Turkish coast.

In March, the European Union reached an agreement with Turkey meant to stem the flow and reduce the number of people undertaking the short but perilous sea crossing to Greece, where many have died after their overcrowded boats sank.

Under the deal, anyone arriving clandestinely on Greek islands from the Turkish coast after March 18 faces deportation back to Turkey unless they successfully apply for asylum in Greece.

But few want to request asylum in the country, which has been struggling with a six-year financial crisis that has left unemployment hovering at around 24%.

The government has been trying to persuade people staying in Idomeni, who include hundreds of families with young children, to leave the area and head to organised camps.

This week it said its campaign of voluntary evacuations was already working, with police reporting that eight buses carrying about 400 people left Idomeni on Sunday. Others took taxis heading to the country's main northern city of Thessaloniki or a nearby town of Polycastro.

On the eve of the evacuation operation, few at the camp appeared to welcome the news.

Abdo Rajab, a 22-year-old refugee from Raqqa in Syria, has spent the past three months in Idomeni, and is now considering paying smugglers to be taken to Germany clandestinely.

"We hear that tomorrow we will all go to camps," he said. "I don't mind, but my aim is not reach the camps but to go Germany."