MAINZ/COLOGNE  - Migrants who commit crimes should lose their right to asylum, German chancellor Angela Merkel said on Saturday, toughening her tone as crowds gathered in Cologne angered by mass assaults on women on New Year's Eve.

Nearly two dozen asylum seekers were among those suspected of carrying out the attacks, police said this week, heightening tensions over immigration and fuelling criticism of Merkel's refusal to place a limit on the numbers of migrants entering the country.

"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 (CDU) party.

"Serial offenders who repeatedly rob or repeatedly affront women must feel the full force of the law," Merkel told journalists in Mainz, promising a reduction over the longer term in the flow of migrants to Germany.

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

About 1,700 police officers were on the streets of Cologne as protesters, including members of the anti-Islam PEGIDA movement, waited for official permission to march through the city.

At a separate left-wing protest, more than 2,000 mostly women gathered close to the train station where many of the attacks, including muggings and sexual assaults, happened.

"No means no. Keep away from our bodies," read one sign held by a demonstrator.

Merkel's conservative party said it wanted to reduce and control migration to Germany, and send those who had been refused asylum home promptly.(Reuters)