The Swedish activist has returned to the German city almost exactly a year since her first visit. She told the audience she failed to understand how politicians could still "behave as though everything were fine."
Greta Thunberg gave a rousing speech to several thousand people as part of a climate protest in Hamburg on Friday.
Thunberg said that she did not understand how politicians could look their children in the eyes as they were stealing their future.
"The decision-makers are still behaving as though everything were fine and science is still being ignored," the Swedish activist told 20,000 protesters, according to police estimates. Organizers estimated 60,000 people.
"This is 2020, and we need to see real action now."
"If enough people get involved in making change happen, change can happen," the 17-year-old added.
The event took place on the eve of a regional election and was attended by numerous politicians, as well as thousands of schoolchildren, students and activists in the St. Pauli district of the northern German city.
Thunberg (center) with a placard reading "school strike for the climate" (in Swedish Skolstrejk för klimatet) during the 'Fridays for future' protest in St. Pauli, Hamburg
Hamburg pays its respect to Hanau victims
The occasion began with a minute's silence for the victims of the shooting spree in the city of Hanau earlier this week. "We are sad, we are angry," said Yavuz Feroglu of the Kurdish umbrella organization Nav-Dem.
A performance from German hip hop group Fettes Brot preceded the Swede's speech.
The participants, who demanded better climate policies, displayed posters and banners with messages such as "We will strike until you take action" and "The earth is on fire."
jsi/aw (dpa, epd)