German riot police on Saturday arrested some 400 protesters as clashes erupted outside a meeting of the right-wing populist AfD party, which is set to adopt an anti-Islamic manifesto amid a rise in European anti-migrant groups.

Left-wing demonstrators burned tyres and threw firecrackers as they tried to prevent delegates of the Alternative for Germany (AfD) party from getting into the congress in the western city of Stuttgart.

The AfD meeting 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 tear gas to hold off protesters, many dressed in black and masking their faces, as officers escorted AfD members into the congress hall.

"No rights for Nazi propaganda," cried one group of protestors, who threw firecrackers at journalists and over 1,000 riot police battling to keep the standoff from escalating further.

The clashes delayed the opening of the congress by more than an hour.

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.


Call to ban minarets

Around 2,400 AfD members were expected at the weekend congress, which comes after 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 Chancellor Angela Merkel, that today "Islam is part of Germany", a country that is home to some four million Muslims.

Another AfD deputy leader, Alexander Gauland, said that "Islam is not a religion like Catholic or Protestant Christianity but intellectually always associated with the takeover of the state".

Aiman Mazyek, chairman of the Central Council of Muslims in Germany, said it was "the first time since Hitler's Germany that there is a party which discredits and existentially threatens an entire religious community".

The challenge for the AfD is that the sharply reduced migrant influx of recent months has deprived it of its core issue.

Unlike in some other European countries, the AfD cannot bank on widespread discontent in Germany, where unemployment is low and the public still trust the government "more than elsewhere," said Timo Lochocki of think-tank the German Marshall Fund in Berlin.

Nonetheless, shifting more openly to anti-Islam rhetoric "may well carry the AfD" into parliament in 2017, said political scientist Nele Wissmann.

A study by the Bertelsmann Foundation think-tank last year found that 57 percent of Germans view Islam as a "threat" and that 61 percent felt that the religion is "inconsistent with the Western world", a level of distrust that is "hard to ignore", said Wissmann.

The AfD was founded at the height of the eurozone crisis by economics professor Bernd Lucke, calling for Germany to leave the euro and return to the Deutschmark.

However, a more hardline right-wing and nationalist faction, led by Frauke Petry, last year deposed Lucke and wrested control, with particular support in eastern Germany.

The party has not yet decided whether it will team up in the European parliament with France's National Front of Marine Le Pen, but AfD ME Marcus Pretzell announced Saturday that he was joining the Le Pen-led grouping in the Strasbourg chamber.