The new president of North Macedonia, Gordana Siljanovska-Davkova, took the oath of office in parliament in Skopje on Sunday but replaced the official name of the country, "Republic of North Macedonia," with the old name, "Republic of Macedonia."
The Greek ambassador left the parliamentary chamber in protest, according to media reports.
Siljanovska-Davkova clearly won the run-off election on Wednesday as the candidate of the nationalist Internal Macedonian Revolutionary Organization–Democratic Party for Macedonian National Unity (VMRO-DPMNE) against the Social Democratic incumbent Stevo Pendarovski.
The VMRO became the strongest force in the parliamentary elections on the same day. Its chairman Hristijan Mickoski is expected to form the next government.
Both Siljanovska-Davkova and Mickoski and other VMRO politicians had announced during the election campaign that they no longer wanted to use the new official country name North Macedonia, but rather the historical name.
The Social Democrats, who had been in power since 2017 and have now been voted out of office, had reached an agreement with Greece on the name change, which was finalized in February 2019. Athens had insisted on this because a region in northern Greece is also called Macedonia.
The name change was a prerequisite for the small Balkan country to become a member of NATO in 2020. It also paved the way for accession talks with the European Union.
The Greek Foreign Ministry declared on Sunday that the swearing-in ceremony was in "blatant violation of the neighbouring country's constitution." Further progress in North Macedonia's path to EU membership depends on the full use of the country's constitutional name, Athens said.