Many Greeks have taken part in violent demonstrations, claiming "Macedonia is Greek."
Thousands of Greeks have clashed with police on the streets of Athens over a deal with the neighbouring Republic of Macedonia about the country's name.
The agreement means Macedonia will be renamed the "Republic of North Macedonia" in order to end Greek opposition to it joining NATO and the EU.
But many Greeks have taken part in violent demonstrations, claiming: "Macedonia is Greek."
As dozens of masked protesters tried to force the closure of the parliament building by throwing stones, riot police broke up the crowds by firing tear gas.
Some protesters hurled petrol bombs and stones at police but others demonstrated peacefully with the estimated 60 thousand demonstrators who gathered in Syntagma Square, outside parliament.
Greeks from all political parties feel strongly about the issue and some of those attending told euronews why they were there:
"We are entitled to a referendum for the name of our neighbouring country in contrast to what the Prime Minister says," said one woman.
"For Macedonia blood was spilt," explained another protester. "We as Macedonians do not concede even a letter from this sacred name."
"Its damaging for our country," one man said. "It will be good to solve the name issue BUT with our terms and for the good of our own country."
Prime Minister Alexis Tsipras is expecting a showdown in parliament over the agreement.
Our correspondent in Athens, Ioannis Karagiorgas, said: "They are demanding a referendum to decide on the Prespa Agreement. They say that they will fight until the end."