German authorities are investigating after numerous women were sexually assaulted and mugged during New Year's Eve festivities in the Cologne city center, according to CNN.

Cologne police told CNN that least a quarter of the 90 criminal incidents reported were sexual assaults, including at least one rape, with the perpetrators described as gangs of Arab or North African men.

Around 1,000 men were involved in the attacks, which took place at the city's central railway station, with women also being targeted in Hamburg, according to the BBC. A volunteer police officer was one of the women targeted.

Wolfgang Albers, who is the city police chief, described the attacks as "a completely new dimension of crime," according to the BBC.

Cologne's mayor, Henriette Reker, organized an emergency meeting on Tuesday in order to prevent the city from turning into a "lawless zone," according to the Guardian.

Police were unaware of the attacks despite their frequency because of the large crowd that New Year's Eve celebrations attract in the city center, according to the BBC.

The attacks occurred after the men broke into groups while police attempted to prevent fireworks being thrown into the crowd, according to Reuters. There are concerns that similar oversights could happen next month in the city's annual carnival celebrations, when the city is shut down with alcohol-ridden street parades and parties, according to Reuters.

German Chancellor Angela Merkel responded to the attacks in a statement from her office, stating that the authorities should find the attackers as "quickly and comprehensively as possible and to punish them without regard to their origin or background," according to the Guardian.

German Justice Minister Heiko Maas tweeted that "we won't tolerate these abhorrent assaults on women," but stated that the crimes were not linked to migrants and refugees and that "all are equal before the law," according to the BBC.