The activists in Berlin glued themselves to the airport runway.
Berlin-Brandenburg Airport closed its takeoff and landing runways on Thursday due to climate protesters who jumped onto a runway and glued themselves to the tarmac, according to Sky News.
The news outlet further reported that the activists from the Last Generation environmental group, who say a 70-year-old man was among those involved, were also seen riding bicycles across the shunting area - bringing air traffic to a standstill.
Video footage shows members of the group climbing through a wire fence that had been cut to access the runway. The group called on the public to stop travelling by air and on the government to stop subsidising it.
"The plane is not a means of transport for ordinary people," Letzte Generation said in a statement. "Most people - about 80 percent of the world's population - have never flown in their lives. One affluent percent of the population is responsible for around half of flight-related greenhouse gas emissions."
According to the American newspaper The Hill, the airport had resumed flight operations on its runways as of Thursday evening local time, but warned that there would still be delays and urged patience.
Letzte Generation has previously engaged in similar stunts. Two of the group's protesters last month threw mashed potatoes at a Claude Monet painting at the Museum Barberini in Germany in an effort to call attention to the climate crisis.
A spokesperson for the airport said the activists had entered the airport from two points in the north and south, and police had detained them, reported the news agency Reuters.
The spokesperson could not say how many flights were affected.
Flight tracking website FlightRadar24 showed that some aircraft that were originally heading to the airport changed their route.
Some 13 flights had been diverted to airports in the eastern state of Saxony by 6 p.m. (1700 GMT), the rbb broadcaster reported, citing Mitteldeutsche Flughafen AG, the company that operates the airports in Dresden and Leipzig.