Germany’s largest airport in Frankfurt was forced to suspend arrivals and departures after a protest by climate activists who blocked runways by glueing themselves to the tarmac.

Frankfurt airport, one of the country’s busiest airports and a major international hub, on Thursday advised passengers to check the status of their flights due to the “ongoing demonstration”.

While operations resumed and all four runways were operational a few hours later, an airport spokesperson said that about 140 flights had been cancelled of 1,400 planned.

The climate activist group Letzte Generation (The Last Generation), which orchestrated the action, said in a statement that six of its members used pincers to cut openings in the airport wire fence before making their way “by foot, with bicycles and skateboards to different points around the runways”.

Protesters also held banners on the tarmac that read, “Oil kills”.

German Interior Minister Nancy Faeser, whose ministry wants to punish unauthorised airfield access with up to two years in prison, called the demonstration “dangerous, dumb and criminal”.

“These acts must be punished more severely. We have proposed severe prison sentences,” she added in a post on X.

Police said eight people were detained, the German news agency dpa reported.

The group, which wants the German government to pursue a global agreement to exit oil, gas and coal by 2030, has listed several countries across Europe and North America where similar disruptions are planned.

The Last Generation is also part of the A22 Network of groups committed to non-violent climate protests, which said it was planning to disrupt airports in several countries in the coming months.

According to Our World in Data, a research organisation, global aviation is responsible for about 2.5 percent of global carbon emissions and has contributed “around 4 percent to global warming”.

Thursday’s protest came a day after similar demonstrations took place across several European airports, including London’s Heathrow, where several activists were arrested on Wednesday.

At Austria’s Vienna airport, four activists poured an orange substance in one of the airport’s halls, holding signs that read, “Oil kills”.

The Last Generation activists also briefly disrupted traffic at Cologne-Bonn airport on Wednesday after they glued themselves to a runway.

Activist Ronja Kuenkler told reporters that Wednesday’s protest was just the beginning and declined to answer whether any action was planned for the Paris Olympics 2024, which begin on Friday.