Refugees have clashed with Macedonian police after trying to scale the fence separating Greece from Macedonia in the border town of Idomeni.
Macedonian police used tear gas and stun grenades in an attempt to keep the migrants at bay, and they responded by throwing rocks at the police.
Greek media reported that plastic bullets were also used. Greek police were standing by, not interfering.
Volunteer doctors were treating several migrants with respiratory problems, Achilleas Tzemos, deputy field coordinator of Doctors Without Borders said.
The clashes began soon after some 500 of the more than 11,000 migrants stranded at a camp on the Greek side of the border responded to rumours that the border was about to open by gathering close to the fence.
A delegation of five migrants asked Macedonian police whether the border was about to open.
When Macedonian police denied the rumours, spread by activists since Saturday, more than a hundred migrants, including several children, tried to scale the fence.
Macedonia and other Balkan countries to its north have shut their borders, closing what was the busiest migrant route to central Europe.
The European Union has since put an end to the hopes of many migrants, saying it would only accept war refugees from Syria and Iraq as well as people from other countries who are considered in need of asylum.