More than 1,000 people were protesting Saturday in Stuttgart, where Alternative for Germany, a far-right political party, is holding a two-day conference over the weekend.
Demonstrators tried to block party members from entering the building where their conference was taking place.
Protesters then clashed with police, throwing flares and setting tires on fire, according to Richard Leuze, the spokesman for Reutlingen police.
About 2,000 party members were trying to attend the conference, CNN reported on Sunday.
Another smaller, peaceful demonstration in the southern German city drew about 800 people.
More protests are expected Sunday during the convention's second day.