At least 35,000 people joined 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 Trans-Atlantic Trade and Investment Partnership 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.
Organizers estimated more than 90,000 people took part in the rally, which police said remained peaceful.
Around 35 farmers, protesting genetically modified produce and advantages for big businesses, led the march in their tractors towards the city's Agricutural Ministry offices, followed by a wooden horse carrying a sign saying "TTIP - a Trojan horse?".
Meanwhile organizers handed out flags saying "Yes we can - stop TTIP," referring to a slogan used during Obama's election campaign.
Merkel defended the agreement in a video podcast released Saturday, saying that the TTIP presented "a great opportunity to define global standards" in trade.
"When two large markets such as the United States and the European Union come together to define standards, it will become very hard for other parts of the world not to abide by them. The opportunities it brings far outweigh the risks," she said.
Organizations for environmental and consumer protection as well as the developing world had called on people to march in Hanover against what they see as a possible reduction in Europe's ecological and social standards as a result of the agreement.
Speakers at the demonstration were quick to say that the march was not about anti-American sentiment. US citizens were among those taking part in the march, holding up a sign reading "Americans against TTIP."
Support for the TTIP - 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 rejects TTIP, 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.