The assailant is a migrant and Islamist known to German security forces.
German Chancellor Merkel condemned the violence but also said the majority were peaceful and legitimate protesters.
The section that needs to be resolved relates to the U.S. insistence that there be a reference to fossil fuels.
Demonstrators smashed shop windows, torched cars and set fires in the streets.
Ahead of Friday’s G20 meeting in Hamburg, there were violent clashes between riot police and ‘Welcome To Hell’ anti-globalization protestors.
German authorities are preparing for 100,000 demonstrators to hit the city.
A foreign ministry official says "those people are not welcome in Germany for the foreseeable future."