Campaigners have targeted the headquarters of a firm connected to budget airline Easyjet as part of a protest against short-haul flights.
Protesters chained themselves to railings outside the entrance
Twelve people from Plane Stupid climbed onto the roof of Easygroup's office in Camden, north London, on Monday morning while another six blocked the entrance.
Protesters, who stayed at the building for about four hours, said they felt they had made their point.
Easygroup said there was little disruption to business.
Protesters scaled the building at about 0650 GMT and unfurled a banner which read 'End Short Haul Flights'.
About 20 Easygroup staff were delayed getting into the building by an hour and a half.
John Stewart, who took part in the protest, said: "Tony Blair talks the talk about taking action on climate change while building more runways and allowing more planes at airports.
"Immediate action should be taken to deal with short-haul flights by increasing the tax charged on tickets for short-haul flights."
Plane Stupid also claimed it had targeted up to 25 UK travel agents overnight, chaining front doors and leaving signs on windows.
James Rothnie, of Easygroup, said: "The airline Easyjet functions out of Luton.
"A lot of my colleagues were asking what the protesters were doing here and I think most were baffled as Easyjet is not based here."
An Easyjet spokeswoman said the firm took its environmental responsibilities "very seriously" and its fleet of 122 aircraft were fuel efficient and used the latest technology.
The action coincides with the start of the UN Climate talks in Nairobi.