London police are investigating after explosive devices were found at two of the city's airports and a train station.

Police said in a statement that three packages containing explosives were found separately at Heathrow Airport, London City Airport and Waterloo station.

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The small improvised devices were found in postal mailers, and appeared to be capable of "igniting an initially small fire when opened," police said.

Nobody was hurt as a result of the devices, which police are treating as a "linked series."

No flights or trains were affected by the discovery of the devices, police said.

An office building at Heathrow was evacuated after a package was discovered there. Police cordoned off part of Waterloo station upon finding another package, and staff members evacuated an area of London City Airport when a device was found there.

London City Airport tweeted shortly before noon EST that all 100 staff members who had been evacuated had returned to their posts safely.