German leaders have voiced outrage over a string of apparently coordinated sex assaults against women in the city of Cologne on New Year’s Eve, lashing out at police for failing to stop the attacks.

Police in Cologne said 90 criminal complaints had been filed by women by Tuesday. According to witnesses, groups of 20-30 young men surrounded women during New Year’s festivities outside the main train station next to the Gothic cathedral, assaulted them and in many cases robbed them.

According to police officials, one rape case has so far been reported as well.

On Tuesday, Chancellor Angela Merkel called Cologne mayor, Henriette Reker, expressing serious concern over the “repugnant” attacks. She also urged an investigation into the incident.

"Everything must be done to find as many of the perpetrators as quickly as possible and bring them to justice, regardless of their origin or background," Merkel's spokesman, Steffen Seibert, quoted her as saying.

Witnesses and police said the assailants were drunk and aggressive and appeared to be of “Arab or North African origin.”

“We assume more people will come forward," police chief Wolfgang Albers said, adding that a plain clothes policewoman was also among the victims.

Interior Minister Thomas de Maziere criticized the police for failing to stop the attacks and demanded an immediate clarification.

Police officials said they had evacuated the area, where most of the assaults took place, over fears that New Year fireworks could harm people.

De Maziere said, "It is not acceptable that the square could be evacuated and then [the attacks] take place" in the same place with police "waiting for complaints" from victims before taking action.

The assaults have fueled debate among Germany’s far-right politicians who are against Merkel’s open door policy that resulted in accepting around one million refugees in 2015 from war-torn countries.

"Is this the 'cosmopolitan and colorful' Germany that Merkel wished for?" said Frauke Petry, the leader of the nationalist party ‘Alternative for Germany’, who has called on the government to decrease the number of refugees.

Justice Minister Heiko Maas, however, warned against anti-refugee scapegoating.

"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. Now is the time to determine the facts and then decide on the necessary consequences,” Maas said.