German riot police have arrested around 400 protesters trying to block access to the congress of the far-right Alternative for Germany (AfD) political party, as clashes broke out between party members and left-wing activists in the German city of Stuttgart.

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.

"No rights for Nazi propaganda," cried one group of demonstrators.

No serious injuries were reported as a result of the clashes and despite the protest and arrests, the conference began as planned on Saturday morning.

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Formed three years ago in opposition to eurozone bailouts, the AfD has morphed into an anti-immigration party over the past year. It has kicked out its founder and seized on a record influx of refugees to lure new voters and steal disaffected members of Chancellor Angela Merkel's conservatives.

At the moment, the party is represented in half of Germany's state legislatures.

Polls put AfD at 20 percent in the east, trailing behind Merkel's Christian Democrats, which means that the party is challenging the Social Democrats. The AfD's popularity has climbed to 13 percent overall in Germany.

Islam 'incompatible with German constitution'

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.

The party's other policies include a call to deport foreigners convicted of crimes and to reinstate military service for young men. In foreign affairs, the AfD wants to build a better relationship with Russia.