German police have scuffled with protesters as tens of thousands of people gathered in Dannenberg, north of the country, promising to block a nuclear waste convoy arriving from France.
Activists are describing the waste as the most radioactive ever.
The heavily guarded train crossed into Germany on Saturday after changing its original route to avoid protesters.
A police spokesman said officers had clashed with around 150 protesters trying to remove the ballast from under the track near Dannenberg.
"Some of them threw stones and firecrackers at officers, who had to respond with batons and pepperspray," but the situation calmed down, the spokesman added.
German Chancellor Angela Merkel denounced the attempted sabotage.
About 16,000 police have been mobilised to deal with protests.