Counter-terror police are investigating three packages containing explosives found at Heathrow Airport, London City Airport and Waterloo station.
The Met’s Counter Terrorism Command is treating it as a “linked series” and “keeping an open mind regarding motives”. No-one has been hurt, the force added.
The Compass Centre in Heathrow’s grounds was evacuated after a package was reported to police at about 09:55 GMT.
The package has since been “made safe”, the Met said.
A Heathrow spokeswoman said the airport would assist the police investigation into the “criminal act”.
Two more packages were found in the capital during the next three hours. An area of Waterloo station was cordoned off after a second package was reported at about 11:40. British Transport Police said the item was “being assessed by specialist teams”.
Staff members were then evacuated from City Aviation House at City Airport in Newham after a third package was reported at about 12:10.
Specialist officers are at the scene and inquiries are ongoing, the Met said.
No flights or trains are believed to have been affected.