The Alternative for Germany (AfD) party is meeting in the western city of Stuttgart, where it is expected to adopt an anti-Islamic manifesto, emboldened by the rise of European anti-migrant groups like Austria's Freedom Party.
The AfD congress comes a week after the far-right Freedom Party's Norbert Hofer sent shock waves through Austria's political establishment by winning the first round of a presidential ballot.
Heavily-armoured riot police used pepper spray to hold off protesters, many dressed in black and masking their faces, as officers escorted AfD members into the congress hall.
Clashes erupted between left-wing activists and AfD delegates, while demonstrators burned tyres and threw firecrackers at journalists and police — who numbered over 1,000.
“No rights for Nazi propaganda,” cried one group of demonstrators.
Now polling around 14 percent, AfD is eyeing entry into the federal parliament in elections next year after a string of state election wins.
The AfD was formed only three years ago and has since gradually shifted its policies to the right, while entering half of Germany's 16 state legislatures and the European parliament.
Having initially railed against bailouts for debt-hit eurozone economies, it has changed focus to protest against mostly-Muslim migrants and refugees, more than a million of whom sought asylum in Germany last year.
The AfD has loudly protested against Chancellor Angela Merkel's liberal migration policy but also channelled popular anger against established political parties and the mainstream press.
Around 2,400 members are expected at the weekend congress, which comes after AfD deputy leader and European parliament member Beatrix von Storch last week caused anger by labelling Islam a “political ideology that is incompatible with the German constitution”.
Von Storch said the congress would call for a ban on Islamic symbols in Germany such as minarets on mosques, the call to prayer and full-face veils for women.
It will openly challenge the government position, repeatedly stated by Merkel, that today “Islam is part of Germany”, a country that is home to some four million Muslims.