Dozens of migrants and refugees were wounded when Macedonian police fired tear gas and rubber bullets at crowds on the Greek side of the border, aid workers say, an act Athens called "dangerous and deplorable".
More than 10,000 migrants and refugees have been stranded at the Greek border outpost of Idomeni since February after a cascade of border shutdowns across the Balkans closed off their route to central and western Europe.
An earlier attempt by a large group of migrants to cross the border fence had resulted in Sunday's confrontation, a Macedonian official said.
Greece said police on the Macedonian side of their joint frontier used tear gas, rubber bullets and stun grenades to push back the migrants. Macedonian authorities would only confirm they had used tear gas.
A deputy field coordinator with medical charity Medicins Sans Frontieres (MSF) told Reuters that, of around 300 people treated, more than 30 had injuries caused by rubber bullets.
A migrant man with a tear gas canister during clashes with Macedonian police during a protest at a fence at the northern Greek border point of Idomeni Source: AAP
A news release issued by MSF quoted one doctor with the charity saying three children had been taken in to its field clinic with head injuries caused by rubber bullets.
Police in Skopje said three officers were also hurt.
The European Union is implementing an accord under which all new arrivals to Greece will be sent back to Turkey if they do not meet asylum criteria.
A Macedonian official who asked to remain anonymous said that a large group of migrants left Idomeni camp on Sunday morning and surged towards the fence.
"They threw rocks at the Macedonian police. The police fired tear gas in response," the official said.
"The migrants were pushing against the fence but standing on the Greek side of the border. The fence is still there, they have not broken through."
Reuters witnesses said a small group of migrants attempted to talk to Macedonian border guards, asking for them to open the border. After given a negative response, they and other migrants started walking towards the fence.
File image of a group of migrant men attempt to tear apart the fence during a protest at the northern Greek border point of Idomeni, Greece. Source: AAP
Macedonian police fired tear gas, and some migrants hurled back some gas canisters and rocks, they said.
In an unusually strong statement, George Kyritsis, a spokesman for migration coordinators in the Greek government, said the use of force was unacceptable.
"The indiscriminate use of chemicals, rubber bullets and stun grenades against vulnerable populations, and particularly without reasons for such force, is a dangerous and deplorable act," Kyritsis said.
More than 50,000 refugees and migrants are stranded in Greece as a result of the border shutdowns.
Achilleas Tzemos, MSF deputy field coordinator, told Reuters more than 300 people had been treated at the makeshift camp.
In addition to more than 30 treated for rubber bullet injuries, over 30 had open wounds and 200 had respiratory problems from tear gas exposure. "Among those with breathing difficulties were quite a few women and children," he said.
