Protesters and police clashed on Saturday morning in the western German city of Essen, the site of a two-day Alternative for Germany (AfD) party congress about to get underway.
A large group of protesters tried to break through a police cordon at around 5:45 a.m. (03:45 GMT), a police spokeswoman said.
Officers used pepper spray and batons to stop their advance, she said.
It was not immediately known whether there were any injuries among the demonstrators.
Around 100,000 demonstrators are expected in Essen.
Authorities have expressed concern that while most of the expected demonstrations were expected to be peaceful, about a thousand left-wing extremists would use violent means to disrupt the congress.
Left-wing groups have indicated that they will block the entrances to the conference centre to prevent AfD delegates from passing through.
About 600 AfD deputies are meeting in the Grugahalle indoor arena in the city. Current co-leaders Alice Weidel and Tino Chrupalla are vying for re-election ahead of German parliamentary elections in the fall of next year.
