Protesters in Cologne | Catiana Krapp for POLITICO Mass protest against AfD conference Far-right party meeting to discuss leadership ahead of September’s election.

COLOGNE — Thousands of protesters took to the streets of Cologne Saturday to demonstrate against the far-right Alternative for Germany, which is holding its party conference in the city.

Organizers said more than 10,000 people took part in the protest which began with a blockade of the hotel near the Rhine where the congress was taking place.

Many of the around 600 delegates needed police protection as they tried to enter the venue where they were planning to discuss the party's lead candidates in September's national election. Two officers were injured, local police said, but police spokesman Christoph Gilles added that the protest was mostly peaceful.

The choice of Cologne was controversial as the city was the site of mass sexual assaults by migrants on New Year's Eve 2015 that gave a boost to Germany's far-right. “It is a clear provocation that this party holds its union in Cologne,” said Hannelore Kraft, minister-president of the state of North Rhine-Westphalia.

Inside the venue, AfD leader Frauke Petry suffered a defeat when delegates voted against discussing her plans to pursue a “realpolitik strategy” and push the party closer to the mainstream.

Petry also criticized more extreme members of the AfD such as Björn Höcke, a regional party leader who caused offense earlier this year with a comment about the Holocaust memorial in Berlin when he said that “Germans are the only people in the world who plant a monument of shame in the heart of [their] capital.”

Petry earlier this week said she would not be the party's lead candidate in the election.

Part of the AfD’s leadership team since 2015, Petry said in a video message on Facebook that she is “not available either as top candidate or as part of a team.”

The convention continues Sunday when they will discuss the election platform and vote on their lead candidates, with German media putting Alexander Gauland (the regional leader in Brandenburg) and Beatrix von Storch (an MEP) among the frontrunners.

Opinion polls suggest the AfD would get around 10 percent of the vote, behind the Christian Democrats (34 percent) and the Social Democrats (30.5 percent).

Catiana Krapp contributed to this article.