As anger mounts over the assaults and the authorities’ handling of the incidents, there is a growing sense of doubt whether Germany, which saw 1 million migrants arrive last year, really “can do it” on welcoming refugees, as Merkel has said many times.

The weekend saw rival demonstrations turn violent.

On Saturday, police clashed with around 1,700 right-wing protestors opposing immigration from Muslim countries.

Police deployed water cannons after demonstrators from the far-right Pegida movement gathered around the Cologne railway station, where the attacks took place on 31 December, throwing bottles and firecrackers.

Pegida protestors chanted for the ousting of Merkel.

They were confronted with around 1,300 leftist demonstrators rallying against nationalism and xenophobia.

Around 500 women also took to the streets to protest violence against women.

But Merkel does not only face criticism from the right.

Germany’s justice minister, Heiko Maas, speculated in an interview with the Bild am Sonntag newspaper that the sex assaults had been planned.

“Nobody can tell me that this was not co-ordinated or prepared …When such a horde meets to commit criminal acts it looks like it was planned in some form,” the Social Democrat (SPD) politician said.

Maas added, however, that it was "complete nonsense" to take these crimes as evidence that foreigners could not be integrated into German society.

The SPD is also supporting Merkel’s proposal for tougher laws on migrant criminals.

Merkel, meeting leaders of her Christian Democratic Union (CDU) in the southwestern city of Mainz on Friday, proposed stricter penalties and faster deportations.

“The right to asylum can be lost if someone is convicted, on probation or jailed,” the chancellor said.

"This is in the interest of the citizens of Germany, but also in the interest of the great majority of the refugees who are here,'' she added.

Under German law, only foreigners convicted of a crime and sentenced to serve more than three years are deported, and only if their expulsion would not endanger their lives.

Meanwhile, Merkel's popularity continues to slide as she refuses to put a limit on the influx of refugees.

While 75 percent of those asked were very happy with Merkel's work in April last year, only 58 percent are happy now, a survey by state broadcaster ARD showed.

Almost three quarters of those polled said migration was the most important issue for the government to deal with in 2016, Reuters reports.

Merkel is set to hold talks with European Commission president Jean-Claude Juncker on Thursday in Berlin, where European migration issues will be high on the agenda.