Three small devices were sent to Heathrow Airport, London City Airport and Waterloo station that "appear capable of igniting an initially small fire when opened," London's Metropolitan Police said.
No one was hurt. Transit on the Docklands Light Railway system to London City Airport was disrupted briefly but resumed soon thereafter. Flights and other transit were not affected.
Authorities wrote in a tweet that the improvised explosives are being treated as linked. They were all found within hours of each other.
The first package was found at Compass Centre, Heathrow Airport's main office, and was opened by staff, "causing the device to initiate," Scotland Yard said in a statement. "This resulted in part of the package burning."
The Compass Centre remains closed after the incident, the BBC reported.
U.K.-based Sky News published images of the packages, including the one that ignited. The photos show A4-size postal bags addressed to each destination.
In a tweet Tuesday afternoon, London Mayor Sadiq Khan expressed thanks to authorities for their handling of the situation and for "their swift actions to keep our city safe."
The Met Police have not ruled out the possibility of other packages. The U.K. Department for Transport encouraged citizens to report suspicious packages.