German leaders have expressed shock over dozens of apparently coordinated sexual assaults against women on New Year's Eve in Cologne blamed on "Arab-looking men", but warned against anti-migrant scapegoating.

Key points: Attackers looked as if they were from "Arab or North African region"

Attackers looked as if they were from "Arab or North African region" Attacks fuel calls from right-wing groups to stop letting in asylum seekers

Attacks fuel calls from right-wing groups to stop letting in asylum seekers Merkel and Cologne mayor promise tough response to attacks

Chancellor Angela Merkel called for a thorough investigation of the "repugnant" attacks, ranging from groping to at least one reported rape, allegedly committed in a large crowd during festivities outside the city's main train station and cathedral.

Police said they had received 90 criminal complaints and quoted witnesses as saying groups of 20-30 young men "who appeared to be of Arab or North African origin" had surrounded victims, assaulted them and in several cases robbed them.

A plain-clothes policewoman was reportedly among those attacked, and the northern port city of Hamburg also reported around 10 similar attacks.

"We assume more people will come forward," police chief Wolfgang Albers said.

Katja, 28, said she was with three friends outside the station when they encountered a group of "foreign-looking men".

"Suddenly I felt a hand on my bottom, then on my breasts, then I was groped everywhere," she told Cologne tabloid Express.

"It was horrible. Although we screamed and flailed about, the guys didn't stop. I was beside myself and think that I was touched about 100 times across around 200 metres."

A woman in her 30s interviewed on rolling news channel N24 said she was groped by a group of "Arab-looking men".

"They didn't look at me aggressively, they seemed more curious than anything, and a little drunk. I was furious," she said.

'This is not about where someone is from'

Police said they evacuated the area because of fears people could be injured by fireworks and admitted the assaults then began without them realising what was happening.

Justice Minister Heiko Maas said the assaults represented "a new dimension of crime that we will have to get to grips with," adding they had appeared to be "coordinated".

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Asked by a journalist whether refugees were behind the rampage, Mr Maas said police were still working to identify the attackers.

"This is not about where someone is from but what they did. Making an issue out of it, lumping it together with the refugee issue, is nothing but exploitation," he said.

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

Germany took in just over 1 million last year, far more than any other European country, and there are almost daily attacks on refugee shelters.

The right-wing populist Alternative for Germany (AfD) party, which hopes to gain seats in three regional elections in March, said the attacks were "a result of unchecked immigration".

"Here we see the appalling consequences of catastrophic asylum and migration policies on Germany's everyday reality," party leader Frauke Petry said.

Merkel promises a 'tough response' to attacks

On Tuesday evening, 200-300 people gathered in front of Cologne's cathedral 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!"

Police said an attack on Henriette Reker a day before she was elected mayor appeared to be motivated by her support for refugees. ( Reuters: Wolfgang Rattay )

Ms Merkel's spokesman Steffen Seibert said she had called Cologne mayor Henriette Reker to express her "outrage" over the violence, which she said required "a tough response from the state".

"Everything must be done to investigate those responsible as quickly and completely as possible and punish them, regardless of where they are from," she said, according to Mr Seibert.

After a crisis meeting, Cologne mayor Ms Reker said new steps would be taken to avoid a repeat, including increasing police numbers at big events and installing more security cameras.

She stressed that women must feel safe at traditional carnival celebrations next month when the city closes down for five days of drunken street parades and parties.

Ms Reker was stabbed in the neck and seriously hurt in October, just a day before she was elected mayor.

Police said that attack appeared to be motivated by her support for refugees.

AFP/Reuters