Police reported that some 3,000 officers were called to the scene as 3,000 left-wing protestors threw stones at the police, and some even attempted to grab the officers' weapons. As a result, some parts of the Old City of Cologne were closed to traffic.
The protest began after demonstrators attempted to stop an "Anti-Islam" march, which was organized by local nationalist party Pro-Cologne. The march was to protest the planned construction of a mosque in the city. The police cancelled the march on concerns of public safety.
Nationalist leaders from Belgium, Austria and Italy arrived in Cologne for a two-day "Anti-Islam conference", with the march protesting the building of a mosque to be one of the highlights, and was intended to support the "1,000 year history of Europe and Western-Christian ideals".
Members of the Pro-Colonge party expected some 1,500 supporters to join the march against the mosque and to protest the "immigration-invasion" of Europe.
The local city council approved the planned mosque to be built at a residential neighborhood. Cologne Mayor Fritz Schramma called the nationalists "agitators and racists, who hide under the guise of a civil movement."
There are about 3 million Muslims currently living in Germany, comprising about 4% of the population.