Police fear the likelihood of violence as left-wing extremists from all over Europe are descending on Hamburg to protest the G20 summit over the weekend.

Ralf Martin Meyer, president of the Hamburg police, has expressed concern that there could be riots and violence at the G20 summit scheduled to begin in the city on Friday. He was also concerned about reports that left-wing extremists from outside Germany are travelling to Hamburg to cause violence Die Welt reports.

“Movements of the autonomous scene in the direction of Hamburg have been observed,” Meyer said. The “autonomous scene” refers to groups like Anti-Fascist Action (Antifa) and other anarchist groups.

“This is not a question of sit-in blockades, but massive attacks,” Meyer said.

Police are on high alert for left-extremist violence after the raids of properties in Rostock which yielded a cache of weapons earlier this week. Among the items found were many knives and bats, cylinders filled with what is thought to be flammable liquids, gas masks and containers full of various chemicals.

Police are currently trying to determine whether or not to charge the two men whose homes were raided with the formation of a terrorist group.

Chief-inspector of the State Criminal Police Office (LKA) in Hamburg Jan Hieber noted that the find was not likely the only cache of weapons belonging to left-extremists. “We can assume that this is only a tiny fraction of what is currently still in cellars and garages in and around Hamburg,” he said.

Hamburg police president Meyer also commented on the finding of the weapons saying, “The militancy of the G-20 opponents is not only a prognosis, it is real, and the cases described demonstrate a high level of criminal energy.”

“The owners were not concerned about sit-in blockades, but about serious and severe injuries. This kind of militancy leaves me stunned,” he added.

Left-wing extremism in Germany has become more and more violent over the past three years growing along with opposition to the pro-migrant policies of the German government. The targets of groups like Antifa and others are often the police or right-wing politicians and their supporters.

The left-extremist scene was recently seen in action during the French presidential election where they attacked police with Molotov cocktails and other weapons.

Some police in Germany have described themselves as being shocked by the rise in violence toward their officers from the extremists, but despite the violence governments like the left-wing coalition in Berlin have chosen to largely focus on right-wing groups.