Extinction Rebellion activists broke into a secure area of Schiphol airport and poured 200 litres of brown liquid on the ground in a protest against the airline industry’s role in climate change.

Questions have been raised about security standards at the airport after 99 demonstrators were arrested by border police inside the secure area beyond passport control.

The Dutch border police (Koninklijke Marechaussee) said there was no threat to public safety because the incident was brought under control. But aviation experts said it highlighted weaknesses in the security system.

“If this group is able to reach secure areas with creative or even illegal means, that is extremely concerning,” Piet Visser, head of the aviation technicians’ union NVLT told the Telegraaf.

One insider added that activists could have easily obtained fake boarding passes on the internet to get through the security gates. “The validation software at the gates isn’t good enough and doesn’t meet EU security rules,” the anonymous source said.

Schiphol airport said it was not clear if the demonstrators had used fake tickets or bought genuine ones to reach the secure zone. Passengers can take up to a litre of liquid in their bags, as long as it is decanted into 100ml containers, a spokesman said.

The Marechaussee said Saturday’s protest would be “evaluated” to see what lessons could be learned, while airline KLM, which was the target of a demonstration by XR earlier this year at its business lounge area, condemned the security breach.

“We strongly condemn demonstrations beyond customs because it jeopardises the safety of our passengers and colleagues. That is unacceptable,” said a spokesman.