Cologne's mayor vowed the city will not become a "lawless zone" after dozens of women were sexually assaulted by a group of 1,000 men described as of North African or Arab appearance on New Year's Eve.

"The mayor cannot and will not accept that a lawless zone develops here," mayor Henriette Reker's spokesman said ahead of a crisis meeting with city and federal police on Tuesday (January 5), Germany's DPA press agency reported.

German police have been accused of a cover up, after police chief Wolfgang Albers only revealed on Monday (January 4) that a group of around 1,000 men responsible for a series of serious sexual assaults outside the city's main train station "from appearance were largely from the north African or Arab world".

There were around 60 allegations of sexual assault, theft, and assault by the group near the city's famous cathedral. One woman was raped, and a volunteer policewoman among those sexually molested. Police are attempting to establish if the group used social media to coordinate and organise the attacks.

A police officer told Cologne's Express newspaper that eight asylum-seekers carrying residency permits had been detained outside the train station after the attacks. Similar incidents have been recorded in the city of Hamburg.

German Justice Minister Heiko Maas tweeted that "we won't tolerate these abhorrent assaults on women – all those responsible must be brought to justice".

Reker's spokesman said that police would be expected to act to guarantee the safety of revellers at the city's carnival, which begins in February. Germany's n-tv reported that police are considering calling in reinforcements from other parts of the country, and installing special surveillance cameras.

Reker, who stood as an independent in municipal elections, was declared the winner as she lay in a coma after being stabbed by a far-right activist opposed to the policy of welcoming refugees to the city.