Dozens of people were hurt Sunday when police fired tear gas at hundreds of migrants as they tried to break through the Greek-Macedonia frontier, where over 11,000 people are stranded.
It was the latest violence to erupt at the flashpoint Idomeni crossing, where huge numbers of migrants and refugees have been camped out since mid-February after Balkan states closed their borders, cutting off access to northern Europe.
Macedonian police accused the crowds of hurling stones and other objects at them in a bid to break down the fence, saying they had used tear gas to protect themselves.
French medical charity Doctors Without Borders said many suffered breathing problems but confirmed they had also treated several people for injuries caused by plastic bullets.
The incident was sparked by fresh rumours that the Idomeni border crossing into Macedonia, largely closed since mid-February, was about to open.
According to a Greek police source, hundreds of migrants had gathered by the fence to demand the border be opened. When they tried to force the barrier, Macedonian police began firing tear gas.
At the scene, protestors with their faces covered with scarves or smeared with toothpaste as a makeshift protection against tear gas could be seen hurling rocks at the fence, an AFP correspondent said.
Part of the fence appeared to have been torn down.
Others ran for cover as tear gas grenades exploded nearby, sending clouds of gas wafting into the air.
"Dozens of people were hurt, mainly suffering respiratory problems, and three had to be taken to hospital," MSF's Achileas Tzemos told AFP.
But he also said several people were injured by plastic bullets, a charge denied by Macedonian police.
"We are not using any kind of bullets as they are forbidden by law in Macedonia. We are not using batons as we are on the other side of the fence," spokeswoman Liza Bendevska told AFP.
"We are using all allowed chemical means."
Earlier, another spokesman said the mob had hurled stones and other objects at police, injuring three of them, and that they had used tear gas to try and break up the protest.
"A large group of refugees attempted to destroy the razor fence and enter Macedonia. They threw stones, metal things and other objects towards police," spokesman Toni Angelovski said.
Macedonian police had initially told AFP it was Greek police who were responsible, but Angelovski later confirmed they too had begun "using tear gas and all allowable means to protect (themselves) and the border".
"No single migrant managed to cross on Macedonian side, but (the situation) is still tense," he added.
An official at a migrant centre on the Macedonian side of the border said three 500-strong groups of people had tried to breach the barrier in three different places.
The makeshift encampment at Idomeni, where people are living in increasingly squalid and overcrowded conditions, has become a symbol of the misery faced by thousands who have fled war and poverty to reach Europe, creating the bloc's worst migration crisis since World War II.
Sunday's incident was sparked by rumours that Macedonia was going to open the border. Similar rumours a fortnight ago also triggered an unsuccessful attempt to rush the fence.
Efforts by the Greek authorities to persuade migrants to leave Idomeni and move to nearby reception centres have not been successful, with many people preferring to stay put in the hope the border will be opened.
In late February, Macedonian police tear gassed some 300 migrants after they broke through a Greek police cordon and raced towards a railway track between the two countries, prompting the European Union to voice "concern" about heavy-handedness.
Sunday's incident came a day after four women and a child drowned off the Greek island of Samos, in the first deaths in the Aegean Sea since a controversial EU-Turkey deal to stem the flow of refugees took effect three weeks ago.
At the port of Piraeus, near Athens, where some 4,500 migrants have set up a huge makeshift camp, several hundred people protested against efforts to move them to dedicated reception centres.