German left-wing demonstrators burned tyres and hurled fireworks and stones this afternoon in clashes with police and AfD supporters as the right-wing party gathered for its congress near the south-western city of Stuttgart, police said.

Some 400 protesters were detained, a police spokesman said.

The demonstrators temporarily blocked access roads to the site of the anti-immigration Alternative for Germany (AfD) party's annual conference, by setting car tyres on fire and forming human chains, police spokesman Lambert Maute said.

The clashes underlined growing social tensions in Germany after a record-influx of more than 1 million migrants arrived here last year alone.

The violence began at around 6 a.m. and clashes continued for four hours, forcing the AfD's two-day congress - which was expected to draw up a party platform encompassing its immigrations views - to open late.

"Police detained around 400 violent protesters who threw stones at officers and attacked them with fireworks," Maute said, adding there were more than 1,000 security forces and about the same number of demonstrators overall.

The police used pepper spray and a water cannon to stop protesters, some of whom were masked, from getting onto the grounds of the conference, but said that some demonstrators still managed to assault several party members.

"There were no injuries reported, only some minor incidents of eye irritation due to the pepper spray," the spokesman said.

One placard from demonstrators read "Your hate campaign pisses us off" while another said "FCK AFD". The demonstrators mostly wore black clothes and some of them had masks, the police spokesman said.

The AfD's anti-immigration and Islam-critical policies were expected to be in focus at the conference. Also on the agenda will be its criticism of the European Central Bank's ultra-loose monetary policy.