The spokesman said public safety was at stake, after police battled with leftists who flooded into the city centre to protest against the rally called by a local group following municipal approval for one of Europe's biggest mosques.
The police spokesman said officers had been attacked by demonstrators who threw stones and tried to steal their weapons. Police had responded with batons.
Some 3,000 police had been drafted in and part of the old city sealed off as authorities appealed for peaceful protests against a weekend congress called by the far-right group Pro-Köln (For Cologne).
Pro-Koeln had hoped 1,500 people would attend Saturday's rally in the city centre to oppose the mosque and an "immigrant invasion" of Europe, due to start at midday on Saturday.
Those attending the congress, including far-right leaders from Belgium, Austria and Italy, were protesting against "Islamification," and voicing support for Europe's "shared, thousand-year history" and "Western values and Christian traditions."
Mayor Fritz Schramma, whose city council gave the green light for the construction of the huge mosque, slammed Pro-Köln as "arsonists and racists" hiding under the cloak of a "citizens' movement" in a speech earlier Saturday.
Counter-demonstrations were called by trade unions, churches and anti-racist movements and were expected to attract between 40,000 and 60,000 people.
Several hundred opponents of the congress on Friday formed a human chain around a mosque in solidarity with the Muslim minority, which numbers more than 3 million in Germany, or 4 percent of the population.
Counter-demonstrators carried signs with slogans including "Stop the Nazi Congress -- Stop Pro Koeln", while some leftist activists threw stones and paint-bombs.
Six leftist protesters and a right-winger were taken into police custody late Friday.