The German far-right party has gathered to elect its leaders, something parties in the country do every two years. On Saturday, the terms of office of leaders Alice Weidel and Tino Chrupalla, who have led the party together for four years, were extended.
Protesters from various parts of Germany blocked major roads and disrupted public transport. According to organizers, at least 50,000 participants turned out, while police said around 31,000.
"The AfD is an anti-democratic party that spreads hatred," 19-year-old protester Lene Krug told the AFP news agency.
The AfD is Germany's largest opposition party at the national level. The country's established parties have repeatedly said they will not cooperate with the AfD.
This weekend's meeting has also sparked controversy because it coincides with the 100th anniversary of a nearby Nazi party meeting that solidified Adolf Hitler's power over the movement.
The years 1933 to 1945 must never be repeated, says 44-year-old protester Ella - who does not want to give her last name.
Thousands of police have been deployed for the two-day congress. Despite fears of violence, the protests have been largely peaceful, apart from minor skirmishes.
