Police in the German city of Cologne say they screened hundreds of men "seemingly of African descent" on New Year's Eve as part of a precautionary measure in reaction to last year's allegations of robberies and sexual assaults.

By early Sunday, police had received reports of two women being sexually assaulted in Cologne. One suspect was arrested.

Cologne's police said in a tweet on Saturday, the men were stopped at two main train stations in the city so that officers could question them and check their identities.

Authorities fielded more than 1,500 officers across Cologne for New Year's Eve celebrations in response to criticism that they failed to stop hundreds of robberies and sexual assaults, blamed largely on men of North African origin, a year ago.

Police installed new video surveillance cameras to monitor the station square in Cologne this New Year's Eve.

"Hundreds of Nafris screened at main railway station," Cologne police tweeted using a colloquial expression for North Africans. With the tweet, the police showed a picture of a large group of men waiting behind barriers.

Last year's attacks in Cologne, where police said the suspects were mainly of North African and Arab appearance, heightened criticism of Chancellor Angela Merkel's decision to allow more than one million asylum seekers into Germany in 2015 and 2016.

This year, police at the flashpoint central railway station and on trains targeted large groups of men of North African origin, checked the identity papers of some 900 people, and ordered many of them to leave the area.

City police chief Juergen Mathis denied criticism on social media that a police focus on men from Maghreb countries amounted to "racial profiling", arguing many had acted in an "aggressive" manner.

"I reject such criticism," Mathis told a press conference. "The clear aim was to prevent similar events to those of last year."

In January 2016, police in Cologne said they had received upwards of 516 criminal complaints from individuals or groups in relation to assaults on New Year's Eve. There were at least 133 charges in the northern city of Hamburg.

The incidents set off a vicious debate over immigration in Germany.