British counter-terrorism police discovered three "improvised explosive devices" near some of London's busiest areas, including Heathrow Airport.
Officers were also called to Waterloo Station and London City Airport. No one was injured and authorities have launched an investigation into the devices, which they believe are connected.
The packages had Irish stamps and Irish police are assisting British counter-terrorism officers.
About 9.55am local time on Tuesday a small package was discovered in Hounslow, not far from Heathrow Airport.
When staff opened it, the device was activated and a part of the parcel was set alight.
"The building is not based at Heathrow Airport and flights have not been affected by this incident," police said. "Specialist officers attended and made the device safe."
Sadiq Khan, the London mayor, tweeted: "Our thanks go to police, security, transport staff and all involved for their swift actions to keep our city safe."