The escalation in tensions that have been simmering for weeks came after more than 500 people gathered at the fence at the sprawling camp of Idomeni, where more than 10,000 migrants and refugees have been stranded since February after a cascade of border shutdowns throughout the Balkans.
More than a million people fleeing conflict poured into Europe mainly through Greece in the past year. The European Union is implementing an accord under which all new arrivals to Greece will be sent back to Turkey if they don't 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 stormed 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."
Aid organizations said they were treating people for tear gas exposure. "We have injuries and are extremely busy," a senior official for medical charity MSF told Reuters. Another aid organization also confirmed injuries among the migrant population.
Reuters witnesses said that unease stirred among the migrant population after a small group of individuals attempted to talk to Macedonian border guards and ask for the outpost to be opened. After they received a negative response, other individuals -- including some with packed bags -- started walking towards the fenced border.
Migrants at Idomeni are demanding that the border with Macedonia be opened, but no migrants have been allowed through for weeks.
Greek authorities have been trying to convince the population to move to reception camps, but migrants have been resolutely refusing to move.