German riot police broke up far-right protesters in Cologne Saturday as they marched against Germany's open-door migration policy after dozens of asylum seekers were arrested for mass assaults on women on New Year's Eve.

The attacks, ranging from sexual molestation to theft, shocked Germany, which took in 1.1 million migrants and refugees in 2015 under asylum laws championed by Chancellor Angela Merkel, despite fervent opposition.

Shortly before Saturday's protest began, Merkel hardened her stance toward migrants, promising expulsion for criminals and a reduction in the influx of migrants into Germany over the longer term.

Police said around 1,700 people attended the rally, which was organized by the far-right anti-Islam Pegida movement, which has seized on the alleged involvement of migrants in the Cologne attacks as proof Merkel's policy is flawed.

Demonstrators, some of whom bore tattoos with far-right symbols such as a skull in a German soldier's helmet, chanted, "Merkel must go," and, "This is the march of the national resistance." One banner read: "Rapefugees not welcome." 

A police spokesman said roughly half of those at the protest were from the "hooligan scene." Some in the crowd threw bottles and firecrackers at officers, and riot police used water cannon to disperse the protesters.

Two people were injured in the clash, and police detained a number of demonstrators, a Reuters witness said.

Pegida, or Patriotic Europeans Against the Islamization of the West, almost fizzled out last year when its leader resigned after a photo was published of him posing as Adolf Hitler.

But its ranks have swelled as resentment of Merkel's welcoming stance to refugees has spread.

In all, about 1,700 police officers were on the streets of Cologne, dwarfing the number on duty during the chaotic scenes of New Year's Eve, when at least 120 women were robbed or sexually molested.

"The events on New Year's Eve led to a lot of emotion," a police spokesman said. "We had feared that emotions would boil over."

About 1,300 people attended a rival left-wing protest in Cologne, according to police.

"No means no. Keep away from our bodies," read one sign held by one of the demonstrators, most of them women.

'We Can Do It'

Merkel's remarks Saturday were in stark contrast to her earlier optimism about the influx to Germany, which has taken in far more migrants than any other European country.

Her 'We can do it' slogan irritated many Germans, uneasy about the mass arrival.

"The right to asylum can be lost if someone is convicted, on probation or jailed," Merkel said after a meeting of the leadership of her Christian Democrats party that was overshadowed by the attacks in Cologne and other cities.

"Serial offenders who repeatedly rob or repeatedly affront women must feel the full force of the law," Merkel told journalists in Mainz.

Under German law, asylum seekers are now typically deported only if they have been sentenced to at least three years in prison, and provided their lives are not at risk at home.

Merkel's conservative party said it wanted to reduce and control migration to Germany, and send those who had been refused asylum home promptly. Such a move would require a change to German law.

"Cologne changed everything," Volker Bouffier, one of the party's most senior members, told the meeting, according to people present.

Earlier in the week, German federal police said they had identified 32 people who were suspected of playing a role in the attacks on women in Cologne. Twenty-two of them were in the process of seeking asylum in Germany.

Police documented 76 criminal acts, most them involving some form of theft, and seven linked to sexual molestation.

Of the suspects, nine were Algerian, eight Moroccan, five Iranian and four Syrian. Three German citizens, an Iraqi, a Serb and a U.S. citizen were also identified.

Similar assaults happened in other cities, such as Frankfurt.