BERLIN (AP) — Leftist protesters are claiming responsibility for torching a half dozen police transport vehicles in Hamburg ahead of this year’s G-20 summit in Germany.

In a message on a leftist forum online, an anonymous poster wrote the attack was meant to “heat up the days before the summit,” the dpa news agency reported Tuesday.

Four police transport vans were completely destroyed in the attack early Monday morning, and two others were damaged.

The G-20 summit is planned for July 7 and 8 in Hamburg, bringing together the leaders of the world’s top industrial and developing nations, including U.S. President Donald Trump.

Authorities are already warning that in addition to large peaceful protests, violent demonstrations are anticipated.