Organisers planned to form a human chain along the 120-kilometre route between Krümmel and Brunsbüttel nuclear power stations to protest government plans to keep them operating.
Three special trains and around 240 buses from around the country brought the protesters to the sites during the late morning, to be met by a group of around 40 farmers driving tractors to Krümmel.
Seven stages for concerts and speeches were set up along the route of the planned chain, which has been organised by a large coalition of citizens' initiatives, environmental groups, political parties, church groups and trades unions.
Related demonstrations are also planned at the Biblis nuclear power station in Hesse and at the nuclear waste storage depot in Ahaus, North Rhine-Westphalia.
The government has recently committed to extending the operating time of the country's 17 nuclear power stations beyond the 2021 closure deadline set in a 2002 law.