European Commission President Ursula von der Leyen called it a "despicable act which goes against everything we believe and fight for in Europe" in a post on the social media platform X.

Charles Michel, the president of the European Council, said he was "outraged" by the "cowardly act of aggression."

Frederiksen, 46, was left "shocked by the incident" on Friday evening in which she was hit while in Kultorvet square, her office said, but further details on possible injuries were not provided.

Police said they arrested a 39-year-old man after the assault and that he would appear in a Copenhagen court on Saturday. They have not said how the attack unfolded. Witnesses told Danish media that the prime minister was able to walk away from the scene.

The incident comes as the EU's 27 countries elect hundreds of members of the European Parliament during four days of voting that began on Thursday. Voters in Denmark go to the polls on Sunday.

There has been a spat of attacks on elected officials at all levels in neighbouring Germany. Mayors, members of the national parliament and local councils, as well as candidates, have been punched, stabbed, spat at and harassed, prompting Chancellor Olaf Scholz to say the country's democracy was under threat.

Last month the EU was stunned by the shooting of Slovakia's prime minister, Robert Fico, in an assassination attempt. The gravely injured leader was taken home from hospital a week ago to continue his recovery there.

Well-wishes, and warnings, pour in

"Mette is a very good colleague and wise partner. I sincerely hope that she is unharmed," wrote Swedish Prime Minister Ulf Kristersson on X. "An attack on a democratically elected leader is also an attack on our democracy."

Roberta Metsola, the president of the European Parliament, wrote: "Tonight's assault on the Danish prime minister is appalling. Violence has no place in politics. Keep strong, Mette!"

Canadian Prime Minister Justin Trudeau posted: "Disturbing to hear that Prime Minister Frederiksen was attacked in a public square in Copenhagen. We cannot dismiss or normalize acts of aggression like this. It's dangerous to our democracies."

Five years as prime minister

In recent days Frederiksen, a centre-left Social Democrat, had been out and about to support her party's top candidate in the European elections, Christel Schaldemose, including on Friday.

However, Schaldemose told Danish news agency Ritzau that the attack was not related to campaign activities.

Frederiksen was elected leader of the Social Democrats in 2015, succeeding Helle Thorning-Schmidt who in 2011 became Denmark's first female prime minister.

At the age of 41, Frederiksen became Denmark's youngest-ever prime minister after besting her main centre-right rivals in the 2019 general election.