Protesters and police clashed on Saturday morning in the western German city of Essen, the site of a two-day party congress by the far-right Alternative for Germany (AfD) that is due to get under way.

Police made several arrests and said that some officers were attacked in violent incidents as thousands of demonstrators sought to blockade the venue and prevent AfD delegates from arriving.

“Some demonstrators wore masks and attacked police officers,” Essen police said on Saturday. Authorities appealed to demonstrators to “keep away from violent actions and troublemakers.”

Several hundred protesters blocked a motorway exit ramp and others occupied streets and intersections near the event hall where the congress is being held.

As many as 100,000 protesters are expected in Essen. Police have deployed thousands of officers as well as water canons to control the demonstrations.

Police clashed with a large group of protesters who attempted to break through a police cordon at around 5:45 am (0345 GMT), a police spokeswoman said.

Officers used pepper spray and batons to stop their advance, she said. Police requested several ambulances, but it was not immediately clear whether any demonstrators were injured in the clash.

Several AfD politicians from Germany’s parliament said that they had been picked up at their hotel by the police and taken to the venue. Some delegates managed to reach the venue on foot undisturbed by protesters.

Around 600 AfD delegates are expected to meet in the city’s Grugahalle indoor arena, with current co-leaders Alice Weidel and Tino Chrupalla seeking re-election ahead of German parliamentary elections in the autumn of next year.

The authorities have raised concerns that, while most of the expected demonstrations would be peaceful, around 1,000 left-wing extremists would use violent means to disrupt the congress.

Leftist groups said in advance they would try to blockade entrances to the venue with the aim of preventing AfD delegates from getting through.

An initial demonstration was held on Friday night, with much larger protests expected throughout the day on Saturday, including a march setting out from the city’s central train station.

The city of Essen had tried for months to prevent the AfD from holding the two-day party conference there, but ultimately failed in court.

A major theme for the AfD’s congress will likely be the party’s second-place finish in the European Parliament elections earlier this month, in which the party gained seats but did less well than pre-election polling had suggested.

The final stages of the campaign were overshadowed by how the party dealt with lead candidate Maximilian Krah, who has been caught up in foreign influence-buying allegations and also by controversial comments he made to an Italian newspaper defending members of the Nazi SS paramilitary.

The AfD was expelled from the Identity and Democracy Group in the European Parliament in response to Krah’s remarks, in which he said not all members of the wartime German SS were criminals.