More than 100 migrants have engaged in running battles with Macedonian police on the other side of a fence on Greece's border with the country.
The skirmishes sent clouds of tear gas wafting over a crowded tent city of stranded refugees and other migrants.
The violence stopped a planned tour of the border fence in Macedonia by the visiting presidents of Croatia and Slovenia.
No injuries were reported from the clashes at the closed Idomeni crossing - while Greek riot police monitoring the stone-throwing migrants on their side of the fence made no arrests, and did little to intervene and retreated during the tear-gas barrage.
Macedonian police fired scores of tear gas canisters, stun grenades and rubber bullets at the protesters, who had earlier tried to scale the border fence using blankets issued by humanitarian groups to get over coils of razor wire.
Many of the canisters were neutralised by blankets and earth thrown over them by the protesters.
About 11,000 people have been living in the informal camp for weeks, since Macedonia closed its border to transient refugees and other migrants hoping to move north towards Europe's prosperous heartland.
Before the shutdown, which was triggered by a similar move in Austria, further north on the migration corridor, about 850,000 people who had arrived in Greece on smugglers' boats from Turkey had entered Macedonia from Idomeni.
The camp residents - mostly Syrian, Iraqi and Afghan refugees - have ignored repeated calls from Greek authorities to relocate to organised camps, and attempted several mass incursions into Macedonia in recent weeks, trying to bypass the fence or break through it.
Alaeddin Mohamad, a 26-year-old law student from Aleppo, Syria, who has lived in the camp for a month, said that the protest started with a peaceful sit-down in front of the fence, and Macedonian police responded with tear gas.
"We don't want to clash. We want the borders to open and get on with our lives," Mr Mohamad said.
"I want to continue my studies in Europe. I will stay here until the border opens. Otherwise, I will die here."
On Sunday, severe clashes between stone-throwing migrants and Macedonian police using tear gas, stun grenades, rubber bullets and a water cannon left scores injured.
The violence increased friction between the two Balkan neighbours - at odds for a quarter-century over Macedonia's official name - with Macedonia accusing Greece of doing nothing to stop the rioters and Athens denouncing Skopje's heavy-handed response.
On the Macedonian side of the border, the presidents of Macedonia, Croatia and Slovenia - whose countries have sent police to help Macedonia guard its border - met in the town of Gevgelija, a few kilometres from Idomeni.
Croatia's Kolinda Grabar Kitarovic said the European Union should clearly define its immigration policy and send a clear message to migrants stranded in Balkan countries who are hoping the old route will reopen.
In comments to the press, she said that the wave of immigration will not stop "until (migrants) got a clear message" on who is eligible for asylum.