At least 160,000 demonstrators converged on Berlin Sunday, according to police, to protest the norm-shattering overtures by Germany's conservatives toward the far right.

Organizers said 200,000 people had turned out to denounce the breach by the Christian Democratic Union (CDU) of Germany's unwritten agreement not to work with the far right at the national level, in place since World War II.

After the rally kicked off just outside the Bundestag, Germany's parliament building, some protesters chanted slogans including "Shame on you CDU" before moving on toward the party's headquarters.

Others accused the CDU and its leader, Friedrich Merz, of having made a "pact with the devil" by seeking the backing of the far-right Alternative for Germany (AfD) to pass an anti-immigration bill.

"(We want to) make as much noise as possible to call for the self-described 'democratic' parties to protect this democracy," protester Anna Schwarz told AFP.

The 34-year-old said she was joining a political rally for the first time as "we can no longer avert our gaze, it's too serious."

The CDU's canvassing for the far-right AfD's support in parliament last week sparked widespread fury in Germany, less than a month ahead of a key snap federal election.

In doing so, Merz, the frontrunner ahead of the 23 February vote, broke the decades-old "firewall" set up in the aftermath of the horrors wrought by Nazi Germany.

'Terrifying'

The two parties successfully passed a non-binding resolution on Wednesday in an attempt to block undocumented foreigners at the border, including asylum seekers.

On Friday, they failed to pass a contentious bill to further restrict immigration — yet the taboo had been broken all the same.

"Friday I was very nervous, I spent the day watching the debates in the Bundestag with friends. To see, live, the CDU speaking, then the AfD clapping and vice versa, it's terrifying," said Oez, who did not give her full name.

The 33-year-old self-proclaimed "queer militant" said she was reassured by the number of Berliners who had turned out for the rally.

"Today, we need to show that there are more of us defending democracy than there are of them," she said.

Merz launched his all-out immigration crackdown after police arrested an Afghan man following a deadly knife attack a week ago against a group of kindergarten children.

He "wants to cut through" the norm of not working with "right-wing extremists in one fell swoop," said left-wing NGO Campact, one of the organizers of protests in Berlin.

Already on Saturday, more than 220,000 people had marched in cities across the country, including Hamburg, Leipzig, Cologne, and Stuttgart, according to figures compiled by public broadcaster ARD.

Trade unions, civil society and human rights groups, churches, and environmental activists were among those to answer the call.

Center-left Chancellor Olaf Scholz has warned that the conservatives and the far right could soon join forces to govern the country, as has been the case in other countries.