At least 25,000 demonstrators took part in protests against the TTIP transatlantic free trade deal in Hanover on Saturday, police said, a day before US President Barack Obama was due to arrive in the northern German city.
Obama is expected to promote the trade pact under negotiation between the United States and the European Union when he joins German Chancellor Angela Merkel in opening the city's industrial trade fair on Sunday.
Farmers drove their tractors towards the city's Agricutural Ministry offices protesting genetically modified produce and advantages for big businesses, while organizers handed out flags saying "Yes we can - stop TTIP," referring to a slogan used during Obama's election campaign.
Organizations for environmental and consumer protection as well as the developing world had called on people to march against what they see as a possible reduction in Europe's ecological and social standards.
Support for the deal - which would create the world's largest free trade area with 800 million people - has eroded among Germans and Americans.
One in three Germans now reject the Trans-Atlantic Trade and Investment Partnership, according to a survey released this week by Germany's Bertelsmann Foundation.
Among US citizens, support for free trade in general is growing, but that does not apply to TTIP. Only 15 per cent favour it, the survey found. That compares with more than 50 per cent in 2014.
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.