About 90 women have reported being robbed, threatened or sexually molested at New Year celebrations outside Cologne's cathedral by young, mostly drunk, men, police said on Tuesday, in events they have described as 'a new dimension in crime'.

Cologne police chief Wolfgang Albers told a news conference officers described the men as looking as if they were from "the Arab or North African region" and mostly between 18 and 35 years old. "We have one complaint that represents a rape," he added.

Integration commissioner Aydan Ozoguz warned against putting foreigners and refugees, hundreds of thousands of whom have entered Germany largely from Middle Eastern war zones, under "blanket suspicion".

Chancellor Angela Merkel expressed shock over the attacks that police said occurred when about 1,000 men split into gangs as officers cleared a square to stop fireworks being thrown from the top of steps into the crowd below.

While politicians also urged people not to become wary of all refugees, the incident fueled calls from right-wing groups to stop letting in migrants.

Germany took in just over a million last year, far more than any other European country.

Cologne mayor Henriette Reker said it was "unbelievable and intolerable what happened on New Year's Eve" but there was no reason to believe those involved in the attacks were refugees.

Justice Minister Heiko Maas said Germany would not accept such attacks which he described as "a new scale of organized crime".

Around 150 people gathered in front of Cologne's cathedral on Tuesday evening to protest against violence against women. One of them held a sign saying: "Ms Merkel where are you? What do you say? This scares us!"