In a day of pro and contra demonstrations over a mosque in Cologne, extreme leftists tried to force their way past German riot police Saturday and enter a city square that was reserved for the use of a far-right "anti-Islamification" group.

Separately, several thousand peaceful demonstrators listened as Cologne's mayor Fritz Schamma stood up outside the city's great cathedral to denounce the far-right rally.

City and national authorities detest the rightists, but say they must allow them German rights of free speech.

Schramma described those who were to attend the far-right rally a couple of hours later as "racists in bourgeois dress" and "the mouldy clique of Euro-Fascism," adding: "I say, there is the door. Go home!"

The planned anti-mosque demonstration has not only inflamed passions in Germany, but in Muslim nations. Iran demanded that Germany prohibit it. Police focussed on keeping the opposed sides apart so that there was no bloodshed.

At Heumarkt, the square set aside for the rightists, the far left attacked roadblocks at several places and scuffled with riot police, but were repulsed.

"We had to crack down hard to avoid something worse happening," a police spokesman said. Reporters saw two men being detained. A police officer was hurt in the face when a firecracker was thrown at him.

Expecting trouble, riot police kept water-cannon trucks that can knock a man to the ground at 30 metres at the ready.

The mayhem also hit rail links. A signal box on the edge of town was set on fire in the early morning by persons unknown, forcing the closure of a main line for several hours and the diversion of 12 national express trains.

The line was re-opened after hasty repairs.

Pro Cologne, a local group which won 5 per cent of votes at the last city-council elections, has invited 1,000 to 1,500 rightists from all over Europe to join it Saturday at a protest against the mosque and what it terms the "Islamification" of the city.

Police banned Pro Cologne from marching Friday evening to multi-ethnic neighbourhoods, saying that riot police would not be able to keep order.

The German Interior Ministry spoke out Friday against the rally, saying the planned gathering of "populists and extremists harms the co-existence that the city and Muslim citizens have striven for."

The mosque, close to a soaring telecommunications tower in a district on the edge of town, is to have a dome 37 metres high and two minarets stretching up 55 metres. It will serve Turkish-speakers whose current mosque is too small for the congregations.