German police used water cannon to disperse around 500 anti-capitalist protesters overnight in the port city of Hamburg where Chancellor Angela Merkel will host leaders of the G20 leading economies in a two-day summit starting on Friday, Reuters reported.

Tens of thousands of protesters are expected to march in the city this week against globalization and what they say is corporate greed and a failure to tackle climate change. German authorities believe around 8,000 demonstrators were prepared to use violence, the interior minister said on Tuesday. Some 20,000 police officers will be deployed.

Hundreds of mainly young left-wing activists gathered and marched on a main street shortly before midnight on Tuesday in the first major protest ahead of the summit, which will be attended by U.S. President Donald Trump, Russian President Vladimir Putin and Turkish President Tayyip Erdogan.

Der Spiegel magazine reported that the Germany army fears protesters will use unarmed drones and that it has deployed a radar to locate any possible aerial intrusions.

A spokesman for the German military declined to confirm the report.