"An operation is under way against violent individuals" who threw petrol bombs and set fire to patrol cars near a police station in the city's Altona district, federal police said on Twitter.
In the west of the city, a "plume of black smoke" was rising, and cars in some areas had been set alight, the local Hamburg police said separately.
Police said demonstrators had blocked several intersections and so-called transfer corridors -- roads designated to help delegations move between meetings.
On Thursday, a planned peaceful march by around 12,000 people protesting against globalisation turned violent.
At least 76 police officers were injured, a Hamburg police spokesman told AFP.
Friday's clashes occurred as leaders from the world's 20 biggest developed and emerging economies were to begin a two-day meeting focussing on trade, terrorism, climate change and other key global issues.
Hamburg, a vibrant port city, is a citadel of leftwing radicals and authorities have long been bracing for possible violence on the sidelines of the summit.
The German police officers' union GdP on Friday hit out at anarchist groups called the Black Blocks, accusing them of "hijacking peaceful demonstrations by tens of thousands of people to deliberately attack" police.
(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)