Greece's interior minister on Friday likened the grim camp holding thousands of refugees on the border with Macedonia to the Nazi concentration camp Dachau.
His comments came as EU leaders were meeting in Brussels to try to reach a deal with Turkey on the migration crisis.
"I do not hesitate to say that this is a modern-day Dachau, a result of the logic of closed borders," Panagiotis Kouroublis said in televised remarks from the squalid Idomeni camp.
"Whoever comes here takes several blows to the stomach," he told reporters, pledging to boost policing and medical supervision in the area.
Also Friday, a group of protesters climbed atop the Acropolis in Athens and unfurled two banners reading "Open Borders" and "Safe Passage - Stop Wars."
Solidarity protests are to be held in Greece on Saturday in main cities, part of a broader European movement to mark the United Nations' anti-racism day.
The Greek government said more than 46,000 refugees and migrants were blocked in the country because of a border shutdown by Macedonia and other Balkan states last week.
Around a third of them are massed in Idomeni, where a makeshift camp initially planned for 2,500 people now holds over 12,000 mostly from Syria and Iraq — and many of them children.
Recurring rains have turned the overflowing camp into a quagmire. Thousands sleep in tiny tents in muddy fields and ditches and queue for hours for food handouts by aid groups.
Dozens of children are suffering from colds and fevers and a private charity said it would donate 2,000 hepatitis vaccines after two such cases were recorded in Idomeni and the Athens port of Piraeus, where around 4,000 people are in another makeshift camp.
On Thursday, a brawl broke out at the port between Syrians and Afghans during a food handout. There were no serious injuries according to the coast guard.
In a bid to avoid further overcrowding, hundreds of refugees who landed from the Greek islands at Piraeus on Friday were boarded on buses and sent to relocation camps near Ioannina, northwestern Greece.