FRANKFURT, Germany — Germany’s busiest airport canceled more than 100 flights Thursday as environmental activists launched a coordinated effort to disrupt air travel across Europe at the height of the summer holiday season to highlight the threat posed by climate change.

Frankfurt Airport said flights were halted for safety reasons after climate activists breached security fences, triggering a response from police, firefighters and airport security officers. All runways were back in operation by 7:50 a.m. local time, it said. About 140 flights have been cancelled so far, but further disruptions are expected throughout the day, the airport said.

Environmental groups said they planned to target airports around Europe this summer to remind people about the link between fossil fuels, such as those used by airliners, and climate change. The groups are calling for governments around the world to end the extraction and burning of fossil fuels by 2030.

“We sharply condemn these unauthorized demonstrations, and we reserve the right to take legal action against the participants,” Frankfurt Airport said. “Their activities pose severe danger to flight operations — possibly putting human life at risk.”

Climate data showed that Monday was the hottest day ever recorded as human-caused climate change continues to drive unprecedented heat and growing weather extremes. Global investments in planet-warming oil and gas are expected to increase by 7% this year, according to the International Energy Agency, despite global promises to slash fossil fuel use.

Flying is considered one of the most carbon-intensive activities, but the aviation industry is expected to grow steadily over the coming decades despite efforts to contain the climate crisis.

The Last Generation group, which organized the Frankfurt demonstration, said six protesters cut holes in the perimeter fence and headed toward the runways on foot, bicycles and skateboards.

It was the second time in as many days that a protest by Last Generation caused disruption at a German airport.

On Wednesday, five protesters glued themselves to a taxiway at Cologne-Bonn Airport, forcing a roughly three-hour halt to flights. That protest resulted in 31 flights being canceled. There were other protests or attempted protests in other European countries.

Climate activists staged similar actions in Finland, Norway, Switzerland and Spain on Wednesday.

At Helsinki Airport, a handful of protesters blocked the main check-in area for about 30 minutes, but police said the demonstration caused no delays to flights or other disruption.

At Oslo’s main Gardermoen airport, three activists managed to enter the runway area early Wednesday, waving banners and disrupting air traffic for about half an hour. Police said there were no major flight delays.

Police in London said Wednesday that they prevented a planned protest at Heathrow Airport that could have had a significant impact on Europe’s largest airport. Seven members of the group known as Just Stop Oil were arrested at Heathrow and three others were taken into custody at other locations as part of an “intelligence-led” operation, the Metropolitan Police Service said in a statement.

One of those arrested in London was Sean Callaghan, 29, who described himself as an environmental educator.

“I’m taking action at airports this summer because it is impossible for me to see a way in which we can inspire and enthuse students about the future of our planet,″ Callaghan said in a video posted on social media.

Last week, the German Cabinet approved legislation that would impose tougher penalties on people who break through airport perimeters.

The bill, which still requires approval by lawmakers, foresees punishment ranging up to a two-year prison sentence for people who intentionally intrude on airside areas of airports such as taxiways or runways, endanger civil aviation, or enable someone else to. Currently such intrusions only draw a fine.