Police detained 500 leftist protesters Saturday outside the venue of the right-wing populist Alternative for Germany (AfD) party's congress in Stuttgart, in clashes that stretched several hours.

The anti-immigrant AfD - whose appeal has soared amid growing discontent over the arrival of 1 million migrants in 2015 and Chancellor Angela Merkel's open borders policy - was to adopt a manifesto for the first time since the party was founded in 2013.

That policy statement is to include the sentence, "Islam does not belong in Germany." Another issue in the two-day convention was the minimum wage. Both are to be addressed Sunday.

During Saturday's session, party delegates voted to oppose immigration into Germany as a source of labour for the economy. A majority voted to oppose European Union membership for Turkey.

A narrow majority of 52 per cent of delegates approved the party leadership's recommendation to dissolve the AfD branch in the state of Saarland, where the chapter was accused of having contact with right-wing extremists.

The meeting, attended by 2,000 AfD members, got of to a rocky start as nearly 1,500 people aimed to disrupt the meeting in the south-western German city. The demonstrators - some masked, others wielding laths and iron rods - blocked a highway with burning tyres, while others set off firecrackers.

Some 800 to 900 protesters who arrived on buses turned combative, but there were no casualties, police said.

One witness said police used pepper spray to disperse the crowds.

More than 1,000 security personnel were deployed to prevent confrontations between AfD members and leftist protesters.

The demonstrators held banners and shouted: "Refugees stay, Nazis go." Others chanted, "Nazis out. Bye bye AfD."

Calm returned by afternoon, police said, with many demonstrators moving to Stuttgart's city centre. Police said around 1,800 people continued the protests there, while organizers put the count at some 4,000.

No riots occurred in the centre, police said, though multiple officers were hit by bags of excrement. There were reports of smoke bombs and flares being set off.

The AfD, which posted double-digit success in state elections in March, has called for the banning of minarets and burqas in Germany, with party leader Frauke Petry reiterating Friday in a radio interview that Islam was a threat to the country.

Petry told Deutschlandfunk that the spread of mosques was an attempt by Muslims to show that their true loyalties lie not with the European nations they live in, but with their faith.

"If your personal faith means you're a Muslim, it still doesn't mean you need a minaret," she said.

A survey by the Emnid polling organization, published Sunday in the Bild am Sonntag newspaper, found that AfD had climed to third among German political parties, topping the Greens. The poll conducted within the last week put AfD at 13 per cent support among respondents, with the Greens at 12 per cent.

Merkel's ruling conservative Christian Democrats (CDU/CSU) were at 33 per cent and their junior coalition partners, the left-leaning Social Democrats (SPD), at 22 per cent.

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