Thousands of Greeks protested in the northern city of Thessaloniki on Saturday over a deal with neighbouring Macedonia to end a decades-old dispute over its name.
Athens and Skopje reached an agreement in June ending years of acrimony between the two countries over the name of the tiny Balkan state, but it has triggered a furious response from many Greeks.
On Saturday, police fired teargas at a group of individuals who broke away from a main group of demonstrators and threw stones at riot police.
State TV showed images of riot police chasing protesters close to barriers erected around a commercial park where an annual trade fair was taking place. Prime Minister Alexis Tsipras was due to deliver a policy address later at the heavily guarded venue.
Police sources estimated the number of protesters at more than 15,000, many waving the white and blue Greek flag.
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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 civilisation.
Thessaloniki is the capital of Greece's northern province of Macedonia.
The country Macedonia has called a referendum on September 30 on the agreement with Greece to change its name to the Republic of North Macedonia. But many Greeks oppose any name that includes Macedonia.
On Saturday, German Chancellor Angela Merkel urged Macedonians to embrace the deal with Greece, to secure membership of Nato and the European Union.