Thousands of police officers are being deployed in Cologne, Germany, in case of violence at left-wing protests against an Alternative fuer Deutschland (AfD) party conference there.
The AfD, a populist right-wing and anti-Islam party, will seek to choose a new co-leader to take it up to a general election later this year.
Demonstrators on Saturday carried banners saying "Block the Nazis".
Police expect 50,000 protesters and have deployed 4,000 officers.
Police chief Juergen Mathias said information indicated that there would be "several thousand left-wing extremists" and "several hundred violent people" among the protesters.
On Saturday morning about 100 people tried to break through a police line on the opposite side of the Rhine river from where the conference is taking place, newspaper Die Welt (in German) reported.
Outside the hotel, 50-60 people blocked access to the venue using a chain of bicycles. Police broke this up, leading to skirmishes with protesters.
At last year's AfD conference in Stuttgart, hundreds of protesters were detained.
- What does the AfD stand for?
AfD co-leader Frauke Petry announced earlier this week that she would not lead the party in to Germany's general election in September.
The party pushed Angela Merkel's governing conservatives in to third place in a regional election last year, and hopes to build on its success by getting its first seats in the federal parliament.
However, all the mainstream parties say they will refuse to form a coalition with the AfD.
Opinion polls show a sharp recent drop in the AfD's popularity - from 15% late last year to between 7% and 11% now.
Its reputation has been dented by a local leader's comments in January that the Holocaust memorial in Berlin was a "memorial of defeat".
Bjoern Hoecke said Germany needed a "180-degree turn" when remembering its past. The party has voted to expel him and he will not be allowed to attend the conference, but some delegates say they will put forward a motion aimed at keeping him in the party.
The Eyes On Events's correspondent Jenny Hill said that while this weekend was officially a party conference, in truth it would be a weekend of crisis talks for the AfD.