0106 GMT May 22, 2019
The demonstrators staged sit-ins to block roadways to the venue in the city center, delaying the opening of the Alternative for Germany’s gathering by about 50 minutes, AFP wrote.
The party has a leadership race this weekend to fill a void created when its best-known figure abruptly quit.
The AfD captured nearly 13 percent of the vote and almost 100 seats in parliament in September's general election – a watershed moment in postwar German politics that left Chancellor Angela Merkel as the winner but still searching for a ruling coalition.
But its co-chair Frauke Petry abruptly quit the party just days after the election to form her own breakaway party. She accused the nationalist party of flirting with far-right extremism.
Some 600 delegates at the two-day congress will vote on a replacement for her as well as a new board, determining the ideological direction of the party.
"The AfD is unable to settle down, it is wrestling with the course it wants to take and power within the party," news website Spiegel Online said.
"The fight over posts and the platform shows that the party is still divided on how sharply rightward it wants to go."
A large pro-refugee march was planned over the weekend supporting Merkel's liberal border policy, which allowed in more than one million asylum seekers since 2015.
The GdP police union called for calm, following clashes with demonstrators in the western city of Cologne during the last AfD congress in April that left several officers injured.
"We expect all participants in the rallies to exercise their right of assembly peacefully," union leader Dietmar Schilff said. "Any violence will lead to the forfeiture of that right."
Launched as a populist anti-euro party in 2013, the AfD has veered sharply to the right since and campaigned for the September election.
It is now represented in 14 of Germany's 16 state parliaments but has been shunned as a potential partner at the national level by the mainstream parties.
However the fractured political landscape has made it more difficult than ever for Merkel, in power for 12 years, to cobble together a ruling majority.
Talks to form a coalition spanning the political spectrum for her fourth and probably last term broke down in acrimony last month.
She is now trying to woo the center-left Social Democrats back into a right-left "grand coalition" government.
If she is successful and averts a snap election, the AfD would become Germany's largest opposition power, strongly boosting its profile.