Only "idiots or traitors" in Poland should be celebrating the victory for the far-right Alternative for Germany (AfD) in the eastern state of Saxony-Anhalt, Polish Prime Minister Donald Tusk said on Sunday.
The latest projections show the AfD on 44%, narrowly missing out on an absolute majority of seats in the state parliament in Magdeburg. However, the party has claimed a mandate to govern the state.
"In Poland, only idiots or traitors can rejoice at the triumph of the AfD in Germany," Tusk wrote.
He said that "quite a few" have gathered in two opposition Polish parties - Confederation, and the former governing party Law and Justice (PiS).
Reaction to the AfD's victory was more sympathetic elsewhere in Germany's neighbouring countries.
The leader of the right-wing Freedom Party of Austria (FPÖ), Herbert Kickl, described the result as a sign of political upheaval in Germany.
"From today, the powerful wind of freedom from 1989 is blowing through Germany once again," said Kickl, drawing a comparison with the fall of the Berlin Wall.
Voters in Saxony-Anhalt have torn down the "undemocratic firewall" against the AfD, according to Kickl. The FPÖ is Austria's largest party in parliament, but remains in opposition.
Kickl said the AfD will "ultimately achieve its definitive breakthrough with Alice Weidel as Chancellor", said Kickl, referring to the party's national co-leader.
In the Czech Republic, Parliament Speaker Tomio Okamura congratulated the AfD on its election success on public broadcasting channel CT.
"Above all, I hope that it will form part of the new governing coalition in the federal state of Saxony-Anhalt," said Okamura, whose Freedom and Direct Democracy party is part of Prime Minister Andrej Babiš's government.