Greek police have fired tear gas at demonstrators in the northern city of Thessaloniki where Prime Minister Alexis Tsipras was scheduled to deliver a speech on Saturday.
Demonstrators protested against the agreement reached by Greece and the Former Yugoslav Republic of Macedonia (FYROM) to resolve a dispute over the Balkan country's name.
In June, Athens and Skopje reached an agreement ending years of dispute between the two countries over the name of the Balkan state. However, it's triggered an angry response from many Greeks.
Protesters holding Greek flags threw rocks and objects against riot police who responded with tear gas.
Police sources estimated the number of protesters at more than 15,000.
A video taken by Euronews' correspondent Akis Tatsis shows protesters running away from police as they fire tear gas into the crowd.
Separate demonstrations, underway in Thessaloniki, planned to coincide with the annual Thessaloniki International Fair, where Tsipras is expected to announce handouts and tax cuts.
Earlier today, German Chancellor Angela Merkel urged Macedonians to embrace a deal with Greece over the country's name at a referendum later this month, in order to secure membership of NATO and the European Union.
Greece has been at odds with Macedonia since 1991 over the name of the Balkan state, arguing it could imply territorial claims over its province of the same name and an appropriation of ancient Greek culture and civilization.