BERLIN — The tensions simmering beneath Germany’s willingness to take in 1 million migrants blew into the open yesterday (Jan 5) after reports that scores of young women in Cologne had been groped and robbed on New Year’s Eve by gangs of men described by the authorities as having “a North African or Arabic” appearance.

Taking advantage of the New Year’s Eve street party, hundreds of young men broke into groups and formed rings around young women, refusing to let them escape, the authorities said. Some groped victims while others stole wallets or cellphones.

Witnesses described the atmosphere around the city’s central train station as aggressive and threatening, with firecrackers being thrown into the crowd in celebration. The women who were attacked screamed and tried to fight their way free, a man who had struggled to protect his girlfriend told German public television.

The Cologne police added that they had received 90 complaints from victims, including one who said she had been raped. No arrests have been made.

In Hamburg, the police said 10 women had reported that they were sexually assaulted and robbed in a similar fashion on the same night.

It was not clear that any of the men involved were among those who arrived in Germany over the past year from conflicts in Syria, Iraq, Afghanistan and elsewhere.

Chancellor Angela Merkel, in a statement, called the assaults disgusting. “Everything must be done to investigate as completely and quickly as possible those who are guilty and to punish them regardless of how they look, where they come from or what their background is,” she said.

The assaults initially were not highlighted by the police and largely ignored by the German news media in the days afterward.

The attacks and the livid reaction to them presented a new political challenge for the chancellor, whose decision to take in refugees from conflict-ridden nations opened the doors to waves of migrants last summer and fall. As the number of asylum-seekers has grown and the challenge of assimilating them has become clearer, Ms Merkel has come under intensifying criticism for failing to anticipate the social and economic costs of her policy.

The descriptions of the assailants — by the police and victims quoted in the news media — as being young foreign men who spoke neither German nor English immediately stoked the debate over how to integrate such large numbers of migrants and focused new attention on how to deal with the influx of young, mostly Muslim men from more socially conservative cultures where women do not share the same freedoms and protections as men.

The assaults set off accusations on the right and among some political commentators that the authorities and the news media had tried to ignore or cover up the attacks to avoid fueling a backlash against the refugees.

Far-right and anti-immigrant groups and other Germans who oppose the influx seized on the attacks, saying they demonstrated the dangers associated with accepting huge numbers of migrants.

“It is time to send a signal,” said Mr Christopher Freiherr von Mengersen, head of the nationalist Pro-NRW movement, based in the state of North Rhine-Westphalia. “We locals can no longer put up with everything that is being routinely swept under the rug based on a false sense of tolerance.”

Even beyond the usual circle of anti-immigration activists, similar concern could be heard over whether the government’s policy had come at too high a price to social stability.

“The government’s loss of control is not only taking place on the borders,” Mr Alexander Marguier, deputy editor in chief of the monthly political magazine Cicero, wrote online. “For whoever gives up control of who enters the country no longer has control over the consequences of this action.”

Ms Henriette Reker, Cologne’s mayor, who was stabbed during a campaign event in October by a German man who opposed her welcoming attitude toward migrants, sought yesterday to play down the links to refugees, after meeting with police, state and city officials.

“There are no indications that this involved people who have sought shelter in Cologne as refugees,” Ms Reker said.

The assaults reported in Cologne were said to have taken place late Thursday in the city’s main train station and the public square in front of it. The station was a central transit point for anyone coming or going from a fireworks display over the Rhine and the bars and nightclubs in the heart of the city, in the shadow of its landmark cathedral.

The police in Cologne say they believe several hundred men, ages 15 to 35 and visibly drunk, were involved in the violence that began when they tossed firecrackers into the crowd that thronged the square.

The police cleared the area shortly before midnight, blocking the main entrance to the station, in an attempt to control the situation, said Mr Wolfgang Albers, Cologne’s chief of police.

In the chaos that ensued after the square was cleared, the men appeared to have moved into the crowds, and that was when the assaults began. “Nobody knew where to go,” Mr Sascha Frohn, who said he was in the station on Thursday, told the public broadcaster WDR. “We stood with our backs to the wall and could see how people were robbed and German girls were groped. I was surrounded by a group of 50 to 60 people from Arabic countries. They would come up to us, shake hands and then try to reach into our bags.” THE NEW YORK TIMES