Cologne, Germany  - Violent demonstrators who thwarted a far-right rally against "Islamification" in Cologne on Saturday continued clashes with police for hours afterwards.

Only 50 supporters of the anti-immigrant group Pro Cologne managed to reach a city square for the scheduled rally against plans to construct a grand mosque in the German city.

An anti-right sit-down by 5,000 mostly peaceful demonstrators had blockaded every entrance to the square.

At the same time, police were fighting running battles with extreme leftists who tried to occupy the square.

The "Stop Islam" rally did start briefly, with more reporters than rightists attending, but then city police declared the event illegal on public-safety grounds.

The counter-demonstrators, including thousands more citizens gathered a kilometre away in support of the mosque, let out a jubilant cheer when the mayor of Cologne, Fritz Schramma, told them the right was beaten.

But the violence was not over. About 6,000 stayed on to hear a pop concert on the street and extreme leftists, who have a deep enmity towards the police, kept scuffling with police or setting garbage alight in the city's alleys.

Baton-swinging police surrounded 500 masked leftists at one point and took the names of individuals who had been seen earlier rioting, throwing stones and firecrackers at officers and trying to snatch away police guns.

The anti-mosque gathering has not only inflamed passions in Germany, but in Muslim nations.

Iran demanded that Germany prohibit it in advance, but German police and lawyers said it could not be banned purely because of the opinions to be expressed. The last-minute ban was justified by the danger to the rightists.

"We can't allow a few hundred people to deliberately enter a battleground," a police spokesman said.

Police chief Klaus Steffenhagen added, "It would have been excessive to deploy water cannon and special units to fight a way into the square for 300 participants."

Pro Cologne voiced outrage at the ban. Its secretary, Markus Wiener, said, "It's typical of the Cologne police leadership that they can't enforce freedom of assembly and that they cave in to street terrorism."

Wiener said his group had had 1,000 supporters trying to attend the rally.

A city councillor for the group, which won 5 per cent of votes at the last poll, said his group would challenge the ban in court. "We'll repeat the event later," Manfred Rouhs told WDR television.

There was no sign in the city of prominent far-rightists from abroad who had been invited, such as Jean-Marie Le Pen, the founder of the National Front in France.

Mayor Schramma, who has personally backed the mosque project, welcomed the rally ban.

"It's a victory for the city of Cologne and a victory by the democratic forces in this city," he told Deutsche Presse-Agentur dpa.

Armin Laschet, minister for minorities in North Rhine-Westphalia state, went further, telling the Tagesspiegel newspaper it was the first time an entire German city "stood up to protect its Muslims."

The mosque, which is to have a dome 37 metres high and two minarets stretching up 55 metres, has been hotly discussed in Cologne, where Turkish-speaking Muslims are a part of what Schramma calls "the city of 180 nations."

Ditib, an Ankara-funded organization that builds mosques all over Germany, says the current mosque at its national headquarters in Cologne is too small for congregations.

Elsewhere in Germany, police confiscated neo-Nazi flags from some of the 270 participants in a march at Dessau organized by the far-right National Democratic Party NPD. (dpa)