BERLIN (AFP) - Forty-four German riot police were injured in clashes with far-right hooligans rallying against "Islamist extremism" in the western city of Cologne overnight, police said Monday.

Police used batons, pepper spray and water cannon against the protesters, who hurled rocks, bottles and firecrackers at them, a spokesman said.

About 20 demonstrators were detained, he added.

At least 4,000 hardline football fans and right-wing extremists from across Germany had massed under the banner "Hooligans against Salafists", state interior minister Ralf Jaeger told ZDF public television.

He said the march turned violent when participants refused to clear the area when their event was over.

Tensions escalated and right-wing protesters repeatedly yelled "Foreigners out!", according to national news agency DPA and witnesses on Twitter.

Hours after the demonstration, a handful of hooligans rampaged through the city centre, the police spokesman said.

The march drew some 500 leftist and anti-fascist counter-protesters, whose demonstration called "Shoulder-to-Shoulder Against Racism and Religious Fundamentalism" went off peacefully.

The "Hooligans Against Salafists" group developed on the Internet and uses social media to draw far-right football fans and skinheads to often violent protests, purportedly against a rise of Muslim extremism in Europe.

Earlier this month, Kurds in Germany clashed with radical Muslims in the northern city of Hamburg and elsewhere, in street protests fuelled by the conflict involving the extremist group Islamic State in Iraq and Syria.