Hundreds gathered with torches and flags as the so-called 'Komiti', supporters of Macedonian sports teams, threw stones and torches at the police.
Greece and Macedonia signed an agreement that could put an end to a decades-old dispute over Macedonia's name.
Under the new agreement, Greece's neighboring nation will be renamed North Macedonia - a move the two countries' leaders said would be the beginning of closer, friendly relations and an example to all Balkan nations.
The deal will also allow Macedonia to bid for NATO and European Union membership.
Since seceding from the former Yugoslavia in 1991, the nation has been provisionally called the former Yugoslav Republic of Macedonia, or FYROM due to the dispute with Greece.
Greece had until now objected to the use of the name "Macedonia", claiming that implied territorial designs on its own northern province of the same name.