Cologne's chief of police dismissed claims of racial profiling Sunday after officers detained hundreds of North African men in an effort to prevent a repeat of sexual assaults during New Year's festivities in the German city a year ago.

The men had gathered late Saturday at Cologne's main train station and in the Deutz district, across the Rhine river. In an overnight tweet, police had described them as being "seemingly of African descent," prompting online criticism that people were being detained based on their appearance alone.

"I reject this negative criticism," Police Chief Juergen Mathies told reporters. "The clear aim was to prevent similar events to previous year."

Hundreds of people were robbed and sexually assaulted during New Year's celebrations in Cologne last year. The crimes were blamed largely on men of North African origin who had taken advantage of chaotic and crowded scenes around the city's cathedral.

Mathies said he had instructed officers to intervene sooner this year. Of the 650 people detained for identity checks, almost all were from North African countries, he said.

"Their characteristics were such that potential crimes could indeed be expected," he added, without elaborating,

About 100 people in all were arrested overnight, while authorities logged about 160 crimes that included almost a dozen assaults or insults of a sexual nature, police said.

Cologne Mayor Henriette Reker said she was "happy and relieved" that the night had passed fairly uneventfully. About 50,000 people rang in the new year in front of Cologne Cathedral, officials said.

Anti-immigration groups seized upon last year's assaults to criticize the government, although police noted that very few suspects came from the same countries as the majority of migrants who arrived in Germany during the previous two years.