About 2,000 migrants have rushed past baton-wielding police who were attempting to block them from entering Macedonia from Greece and several people were injured in the commotion.
The tumult started when police decided to allow a small group of migrants with young children to cross the frontier and crowds in the back squeezed them towards the shielded police wall.
Many women, at least one pregnant, and children fell to the ground apparently fainting after squeezing past the cordon.
Then, thousands of others used the moment to run across a field not protected by barbed wire to enter Macedonia. Police fired stun grenades but did not manage to stop the rush.
There were no immediate reports of the number and extent of injuries. Several children also lost their parents in the chaos and were left shouting "mama, baba".
It was the second day of clashes between the migrants and Macedonian police who are attempting to block them from heading north towards the European Union.
On Friday, police fired stun grenades and clashed with the migrants who tried to rush over the border, a day after Macedonia's government declared a state of emergency on the frontier to stop the human tide. At least 10 people were injured in the melee.
Both Greece and Macedonia have seen an unprecedented wave of migrants this year, most fleeing wars in Syria, Afghanistan or Iraq.
More than 160,000 have arrived so far in Greece, mostly crossing in inflatable dinghies from the nearby Turkish coast - an influx that has overwhelmed Greek authorities and the country's small Aegean islands. Some 45,000 crossed through Macedonia over the past two months.
Few, if any, of the migrants want to remain in Greece, which is in the grip of a financial crisis.
Most head straight to the country's northern border with Macedonia, where they cram on to trains and head north through Serbia and Hungary on their way to the more prosperous EU countries such as Germany, the Netherlands or Sweden.