Up to 10,000 refugees and other migrants are camped on Greece's northern border with Macedonia, waiting for permission to continue their long trek north to seek asylum in wealthier European countries.
Border officials have not allowed anybody to pass for 24 hours, citing a similar policy by Serbia.
Greek police estimate up to 10,000 people - mostly Syrian and Iraqi refugees - are stuck at the country’s Idomeni border in deteriorating conditions.
Hundreds of tents filled the fields stretching towards the border fence, which is patrolled on the Macedonian side by scores of police.
During the day on Monday, Macedonia let in only 30 refugees.
The Idomeni crossing has become a key flashpoint in Europe’s migration crisis.
Several European nations, led by Austria, have imposed refugee caps and border restrictions over the past 10 days, creating a huge backlog of migrants in Greece.
These unilateral actions have infuriated Greece and threaten to damage the unity of the 28-nation European Union.
Some migrants have been waiting at Idomeni for more than a week. The camp is full and hundreds more people arrive daily.
Razor-wire fence
Jasmin Rexhepi, head of the aid group Legis which has volunteers working in Macedonia on its borders with Greece and Serbia, said Macedonian authorities were restricting the numbers of migrants they let through because Serbia only allowed 30 people to cross their border on Monday from a train carrying 410 people. He said Macedonia was waiting for Serbia to open that border.
About 70 people including children, who Macedonia says are mostly from Pakistan, have been stuck on the Macedonian side of the border between two razor-wire fences for three days.
Mr Rexhepi said Macedonian authorities had been trying to send them back to Greece because they had crossed the border illegally, but Greece was refusing to take them back.
“We are providing food, water, sleeping bags and raincoats for this group,” Mr Rexhepi said.
On the Greek side, another group of 150 people who have been told it is their turn to enter Macedonia have spent days in a large tent in front of the crossing.
"I've been at Idomeni for 10 days and it's the fourth day I've been waiting to cross over," said Hassan Rasheed (27) from Iraq.
“Conditions are very bad. There are many ill children who are coughing, and we spent the night in this tent under heavy rain.”
Macedonia closed its border following clashes on Monday when hundreds of migrants tried to force their way into the country. Police responded with tear gas and stun grenades, driving the refugees back.
PA