Organizers of a mass rally in Hanover against the TTIP free trade deal were expecting up to 50,000 people to take part in protests on Saturday, a day before US President Barack Obama was due to arrive in the northern German city.
Obama is expected to promote the transatlantic agreement when he joins German Chancellor Angela Merkel in opening the city's industrial trade fair on Sunday.
Organizations for environmental and consumer protection as well as the developing world have called on people to join in the march, from 12 pm (1000 GMT), against what they see as a possible reduction in Europe's ecological and social standards.
"Obama and Merkel are coming: stop TTIP and CETA!," organizers said will be the motto of Saturday's protest. CETA refers to the Comprehensive Economic and Trade Agreement between the EU and Canada.
The 13th round of TTIP talks is due to take place in New York next week. Negotiators on both sides of the Atlantic have expressed hope in concluding the talks under the Obama administration, but time is running short with US elections due in November.