More than 100 flights have been cancelled.
Flights at Germany's busiest airport were temporarily suspended on Thursday when climate activists glued themselves to the tarmac, with more than 100 flights cancelled.
Federal police reported that several demonstrators entered the airport early in the morning.
Climate activist group Last Generation said that six protesters cut through the perimeter fence at Frankfurt Airport and approached the runways by foot, bicycle and skateboard.
Flight operations began "gradually resuming" shortly before 8am and the airport announced on X that all runways were back in operation shortly after.
Around 140 flights, approximately one-tenth of the scheduled flights for Thursday, were cancelled.
The incident was the second consecutive day of protests by Last Generation. Yesterday they took to the runways at Cologne-Bonn, Barcelona, Helsinki and Oslo airports.
Why are climate activists targeting airports?
Protesters are demonstrating at airports around Europe this week under the campaign 'Oil Kills'.
They say that they are "demanding our criminal governments phase out oil, gas and coal by 2030 by signing a binding international treaty".
Through coordinated actions around Europe, they are asking "our governments ... to stop extracting and burning oil, gas and coal by 2030 as well as supporting and financing poorer countries to make a fast, fair, and just transition".
Second consecutive day of incidents
On Wednesday, five protesters glued themselves to a taxiway at Cologne-Bonn Airport, resulting in a three-hour flight halt and the cancellation of 31 flights.
The same day, flights were also disrupted at Geneva and Zurich airports in Switzerland.
In recent years, airport protests have frequently disrupted flights. Last week, the German cabinet approved a bill to impose stricter penalties on individuals who breach airport perimeters.
The proposed legislation, which still requires approval from MPs, includes penalties of up to two years in prison for intentionally intruding on restricted airport areas such as taxiways or runways, endangering civil aviation, or assisting others in doing so.
In some cases, a sentence of up to five years would be possible. Currently, such intrusions can only result in fines.