(CNN) -- Clashes broke out Saturday as far-right groups from across Europe gathered in Denmark for a rally they said was meant to make their governments act against the threat of Islamic extremism.
Violence flared in the port city of Aarhus as a small group of far-left protesters broke off from a march staged to counter the right-wing rally, said Georg Husted, a spokesman for Aarhus police.
Protesters carrying masks pulled up paving stones and started throwing them and other missiles, he said.
One police officer suffered minor head injuries after he was struck by a bottle, Husted said, but no civilians were hurt or damage caused to buildings.
Police arrested 25 people following the violence, said Husted, most of them from extreme left-wing groups.
Between 200 and 300 people attended the far-right rally, according to police estimates, while about 5,000 people gathered for the anti-fascist demonstration.
The left wing march was peaceful apart from the actions of a few "hard core" protesters who wanted to cause trouble, Husted said.
The far-right rally brought together protesters from the Danish Defense League, as well as smaller groups from Scandinavian countries, Germany, Poland, Eastern Europe and the United Kingdom.
Those attending want to send a "clear message to the leaders of Europe," according to the English Defence League (EDL), one of the organizers of the far-right event.
"Our governments and our media behave as if Islamic extremism exists only in the head of a few extremists, and claim that it is unfair to make the connection between Islam and extremism," an online EDL statement said.
"This is ridiculous, just as it is ridiculous to claim that anyone who criticizes Islam must be an extremist in their own right. We believe in fair criticism of Islam and in the defence of our cultures, our nations, and the rights and freedoms that they have long protected."
Husted said ahead of the rally that police were focused on making sure the day passed peacefully without confrontations between the groups.
"We are going to support anyone who is going to talk and to demonstrate for what they feel, according to Danish law," he said.
In its statement, the EDL said it does not believe that "the way to combat extremism is with more extremism." Racists and neo-Nazis are not welcome, and those supporting the rally have signed an agreement against extremism, it said.
The British government has previously banned some marches by the EDL, whose members are widely seen as holding extremist views.