"Due to an ongoing Demonstration at the airport flights are currently suspended. Passengers are asked not to go to the airport for the time being," the air hub's operator said in a statement.
Air travelers were told to check the status of their flight before coming to the airport and allow more time for their journey.
A federal police spokesperson told the Frankfurter Allgemeine daily that six activists from the Last Generation movement cut through the outer fence to gain access to the airfield and glued themselves to the tarmac, paralyzing all four runways.
Air traffic resumed later in the morning, although the airport did not rule out further flight delays and cancellations.
"Update: runways @FRA operational again since 7:50 AM [04:50 GMT] following closure due to climate protests," the airport authority said.
The demonstration caused at least 140 flight cancellations. Some 50 flights were rerouted.
German Police Bolster Airport Security After Climate Protests Hit Runways
The German police are beefing up security at the airports in response to disruptive climate protests, media reported on Thursday.
Earlier in the day, Last Generation activists got onto the runway at Frankfurt airport and halted all flights. The movement posted a photo of a protester being glued to the tarmac and holding up a poster that read "oil kills."
The police are strengthening security measures at all major airports in anticipation of further disruption by climate protests, the Bild newspaper reported.