A crowd of migrants has broken down a barbed-wire fence on the Macedonia-Greece border using a steel pole as a battering ram.
TV footage showed migrants pushing against the fence at Idomeni, ripping away barbed wire, as Macedonian police let off tear gas to force them away.
A section of fence was smashed open with a metal signpost. It is not clear whether any migrants got through.
Many of those trying to reach northern Europe are Syrian and Iraqi refugees.
About 7,000 people are stuck on the Greek side of the border, as Macedonia is letting very few in. Many have been camping in squalid conditions for a week or more, with little food or medical help.
The chaos on Monday erupted as hundreds of people pushed their way past Greek police to reach the gate used to let trains through at the border crossing.