German riot police on Saturday briefly detained some 500 protesters, as clashes erupted outside a meeting of the right-wing populist Alternative for Germany (AfD) party, which is set to adopt an anti-Islamic manifesto amid a rise in European anti-migrant groups.

Left-wing demonstrators tried to break up the start of the AfD's weekend congress in the western city of Stuttgart. Police counted up to 2,000 left-wing protesters, some of whom burned tires and hurled stones and fireworks to try to stop the AfD's congress going ahead in Stuttgart.

The AfD gathering 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 armored 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 protesters that threw firecrackers at journalists and over 1,000 deployed riot police. The clashes delayed the opening of the congress by more than an hour.

After the congress started late, more than 2,000 AfD members listened to their party leaders' call for an end to Angela Merkel's refugee-friendly politics and a return to Christian values. "We always wondered when the brave child would finally appear to voice the thoughts of the silent majority and declare that the 'Chancellor of no alternatives' is nothing but naked," said party leader Frauke Petry, 40, in her opening speech. "And I think, this brave child is us," Petry added.

The violence began around dawn and clashes continued for several hours. Police used pepper spray and threatened to use water cannons to stop protesters, some of whom were masked, from getting onto the grounds of the conference. Some demonstrators still managed to assault several party members, they said. Stuttgart police said on Twitter that the more than 500 people taken into custody were later released "in small groups." Three officers were lightly wounded in the hours-long scuffles, but there were no reports of injuries among the demonstrators, it added in a statement.

Now polling around 14 percent, the 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 Merkel's liberal migration policy but also channeled popular anger against established political parties and the mainstream press.

Around 2,400 AfD members were expected at the congress, which comes after deputy leader and European parliament member Beatrix von Storch last week caused anger by labeling 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 Merkel's claim that "Islam is part of Germany," a country that is home to around four million Muslims.

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 that 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 Petry, last year deposed Lucke, 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.

Support for far-right populist parties across Europe has been surging especially over fear of a refugee influx fleeing violence and persecution in the Middle East and Africa. Anti-immigration sentiments and xenophobia have become more visible in the country as a result of the efforts of political parties like the National Democratic Party of Germany (NPD) and the AfD as well as anti-Islam and anti-immigrant initiatives like the Patriotic Europeans Against the Islamization of the West (PEGIDA) that have staged anti-immigrant rallies and drawn thousands of people throughout Europe. Despite warnings from other European political figures, Islamophobia and xenophobic rhetoric still survive and have gained ground across Europe.