Around 100 people took part, organised by pressure group Extinction Rebellion, large groups sitting in front of three entrances to block access to the building for several hours.
Extinction Rebellion describes itself as an international movement using non-violent civil disobedience “to achieve radical change in order to minimise the risk of human extinction and ecological collapse”.
As musicians played, protesters wearing armbands with the Extinction Rebellion logo sang songs and danced around with white, cloud-shaped placards and banners with slogans like “Be part of the solution, not pollution”.
“We are facing a total climate emergency,” Extinction Rebellion spokesman Micael Metry told AFP.
“Private jets emit 20 times more CO2 per passenger than normal airplanes,” he said.
“It is very important for us to denounce this completely absurd and unjust means of transportation, which is used by a tiny fraction of the population.”
Sonia Ediger, who said she had come from Lausanne to take part in the protest, called on the “powerful people of the world” who fly private jets “to come down out of the clouds”.
“We are seeing the world collapse around us, we see catastrophe after catastrophe, ever bigger, ever more frequent, all around us,” she told AFP, insisting that “radical change” was needed.
A large number of Geneva police, some in riot gear, assembled to monitor the unauthorised protest, but kept their distance for several hours.
At mid-afternoon, police asked the demonstrators to identify themselves and then leave in small groups, which they did peacefully.
Police spokesman Silvain Guillaume-Gentil told the ATS news agency they had not yet decided whether to bring charges.