Greek police have scuffled with hundreds of migrants who gathered near the northern city of Thessaloniki hoping to enter North Macedonia.
The trouble flared outside a migrant camp on Friday after a social media rumour suggested onward travel restrictions had been lifted.
Meanwhile in Athens, dozens of migrants protested at the capital's Larissa station, disrupting rail services.
Many of the migrants are refugees from the Middle East.
Reports say some migrants outside the official Diavata migrant camp hurled sticks and stones at police, who responded with tear gas.
Greek media describe as "fake news" the Facebook story about plans for an organised crossing of the North Macedonia border.
About 600 spent the night camping in a field outside the Diavata camp, and there were more clashes in the morning as police blocked the route to the north.
More than 100 tents were reportedly pitched in the field by Friday morning.
The Diavata camp, one of three temporary reception facilities on the Greek mainland, has capacity for 936 people, according to Asylum Europe.
Bilal Jaf, a 25-year-old Kurdish migrant from Iraq, told the BBC by phone that the situation in the camp was tense, adding: "We're afraid that the police will try to evacuate our makeshift camp.
"I have been living in Greece for 11 months, waiting for my asylum request to be examined. I don't know for how long I should wait for that."
Karzan Abdullah, 24, also an Iraqi Kurd, said: "I have lived in Greece for 12 months. I have to go to Europe, because there is no life here anymore.
"We are informed that the Greek-North Macedonian border will open for us today. My friends, who also want to join the caravan, have been blocked by the police at a railway station in Athens."
Legacy of 2015 crisis
Tens of thousands of migrants remain in overcrowded camps in Greece, having arrived there in huge numbers in 2015-2016. They include many Afghans, Syrians and Iraqis.
In that crisis Germany took in more than 800,000 asylum seekers - an issue that remains very controversial. International law grants a right of asylum for refugees fleeing war or persecution.
The influx declined rapidly after Turkey reached a deal with the EU to intercept migrant boats and Balkan countries imposed tight border restrictions.
In 2016, a sprawling tent city formed at Idomeni, on the Greece-North Macedonia border. It was later cleared by Greek police and the migrants were redistributed to various official camps.