By Phoebe Fronista and Fedja Grulovic

IDOMENI, Greece (Reuters) – Greek police on Tuesday started moving some of the 8,000 migrants and refugees stranded in a makeshift camp on the sealed northern border with Macedonia to state-run facilities further south.

Several busloads of people, most of them families with children, left the sprawling expanse of tents at Idomeni early on Tuesday and about a dozen more buses were lined up ready to take more, Reuters witnesses said.

At the latest tally, about 8,200 people were camped at Idomeni. At one point more than 12,000 lived there after several Balkan countries shut their borders in February, barring migrants and refugees from central and northern Europe.

Greek authorities said they planned to move individuals gradually to state-supervised facilities further south which currently have capacity of about 5,000 people. The operation is expected to last several days.

“The evacuation is progressing without any problem,” said Giorgos Kyritsis, a government spokesman for the migrant crisis. People would be relocated “ideally by the end of the week,” he said. “We haven’t put a strict deadline on it.”

A Reuters witness on the Macedonian side of the border said there was a heavy police presence in the area but no problems were reported as people with young children packed up huge bags with their belongings.

Media on the Greek side of the border were kept at a distance. Inside the Idomeni camp, police in riot gear stood guard as people from the camp boarded the buses, footage by the state broadcaster ERT showed. Some 1,100 refugees and migrants had been relocated by noon, Greek police said.

A police official said about 1,000 people were blocking the sole railway tracks linking Greece and Macedonia. Protesters demanding passage to northern Europe have for weeks blocked the route, forcing trains to divert through Bulgaria to the east. Some goods wagons have been stranded on the tracks for weeks.

The United Nations refugee agency (UNHCR) urged Greece to refrain from using force during the transfer of the migrants and refugees.

“It’s important that organised movements are voluntary, non-discriminatory and based on well-informed choices by the individuals,” spokesman Adrian Edwards told a briefing in Geneva.

International charity Save the Children said it was concerned about a lack of basic services such as bathrooms and shelters in the official camps.

“Many of the children, especially lone children, have been through enough trauma already,” said Amy Frost, team leader in Greece.

“Now that the evacuation has started, it is paramount that authorities make it a priority to keep families together, and to ensure that children are being transferred to facilities where they can live in conditions that meet European and international standards for child welfare,” she said.

(Additional reporting by Angeliki Koutantou in ATHENS and Stephanie Nebehay in GENEVA; Writing by Michele Kambas and Karolina Tagaris; Editing by Raissa Kasolowsky)