Some 15,000 people took to the streets of Duisburg on Saturday, police said, to show their support for diversity and opposition to far-right extremism.
That far exceeded the 5,000 who were registered for the gathering, a police spokeswoman said.
The German Trade Union Federation (DGB) had called for the demonstration for social cohesion and against exclusion, hatred and hate speech.
The union was joined by a broad alliance of political parties, charitable organizations, clubs and associations to send a "clear signal for a vibrant democracy," the DGB said in a statement.
Thousands of people have been demonstrating nationwide in favour of democracy - and against the far-right Alternative for Germany (AfD) - for weeks.
The protests were triggered by a report by a media outlet about a radical far-right meeting in Potsdam in November, attended by AfD lawmakers as well as from the centre-right Christian Democrats.
They discussed a secret "masterplan" to deport migrants on a large-scale. Ever since people have begun to take the streets in growing numbers.
Welfare organizations in Bochum and Herne have called for the formation of a human chain on Sunday, under the banner "Side by side for democracy and humanity." The aim is for participants to form a 7-kilometre link between the two cities' town halls.
Meanwhile, Family Minister Lisa Paus called for more support for those who are campaigning for democracy, tolerance and against far-right extremism.
Stable support structures are needed, Paus said on Saturday as a guest speaker at the state party conference in Cottbus.
She paid tribute to the federal "Live Democracy!" programme but given the precarity of financing, she called for a law to promote democracy as an instrument.
The government has launched a draft law on the promotion of democracy, aiming to better finance associations and organizations that work to strengthen democracy, social diversity and the prevention of extremism.
