Two policemen were injured and a police car was torched during protests in Cologne at a meeting held by the anti-immigration Alternative for Germany (AfD) party.
Around 50,000 demonstrators were expected at the party congress to discuss policies for September's national elections, while police deployed 4,000 officers.
During tense standoffs, around 50 people clashed with police on horseback, said Broadcaster n-tv, while police used pepper spray on the protestors, a TV correspondent said.
A Cologne police spokesman said the situation was "fairly aggressive", while an iron bar was thrown at an AfD delegate said a BBC reporter.
Local residents tweeted that local shops were closed for the day, while police helicopters circled overhead.
As 600 party delegates walked towards the hotel venue in Cologne, demonstrators attempted to stop them passing through the metal barriers, lead to confrontations with authorities.
"We have information that several thousand left-wing extremists will be coming to Cologne…including several hundred violent people," police chief Juergen Mathias said.
One police officer suffered a facial wound, leading to one male protestor being placed under arrest.
After weeks of bitter infighting, co-leader Frauke Petry, who is pregnant with her fifth child, made the shock announcement that she would not seek to lead the AfD's campaign.
"(Petry) has been unable to stop the AfD, which began as a party of euro critics, from becoming more and more a catch-all for racists, right-wing nationalists and the far right," Cologne's daily Stadt-Anzeiger said. "The fight within the AfD rages on."
Opinion polls show the AfD at between 7% and 11%, a slump from the 15% support it had last year, AFP reported. All the mainstream political parties have refused to form a coalition with the anti-Islamist AfD.
Political analysts say the majority of the AfD's core supporters are on the far right of the political spectrum. There seems to be little appetite for most German voters on radical policies, particularly after the wake of President Donald Trump's victory in the US and Britain's vote to leave the EU.
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