Police received separate reports of suspicious packages at Waterloo Station and Heathrow and London City airports earlier today.
Counter-terror police have said three suspicious packages found near transport hubs across London earlier today were "small improvised explosive devices".
Officers were called to Waterloo Station, London City Airport and a building near to Heathrow Airport in separate incidents within hours of each other earlier today.
Three A4-sized white postal bags containing yellow Jiffy bags were found, the Met Police has now confirmed.
Specialist officers have assessed the packages, which were found to be "small improvised explosive devices".
Officers say they are treating the incidents as "a linked series" but are "keeping an open mind regarding motives".
In a statement, the force said: "The Met Police Counter Terrorism Command has launched an investigation after three suspicious packages were received at buildings in London.
"The packages - all A4-sized white postal bags containing yellow Jiffy bags - have been assessed by specialist officers to be small improvised explosive devices.
"These devices, at this early stage of the investigation, appear capable of igniting an initially small fire when opened.
"The Met Police Counter Terrorism Command is treating the incidents as a linked series and is keeping an open mind regarding motives."
Police are still assessing the situation at Waterloo and at City airport, but say a package found at Heathrow has been "made safe".
They have not yet said whether there is a link between the reports.
A staff building at London City Airport was evacuated as a precaution but the airport tweeted to say there was no impact on flights.
Train services at Waterloo are still running as normal.
"At approximately 09.55, police received a report of a suspicious package at The Compass Centre, Nelson Road, Hounslow," the force said of the incident at Heathrow.
"The building was evacuated as a precaution. Specialist officers attended and the package was made safe.
"At approximately 12.10 today, police were called to a report of a suspicious package at London City Airport Aviation House, Royal Docks, Newham.
"Specialist officers are at the scene. The building has been evacuated as a precaution. Enquiries are ongoing."
In a statement about the incident at Waterloo, British Transport Police said: "We were called to Waterloo station at 11.40am today following reports of a suspicious package.
"The item is currently being assessed by specialist teams.
"Cordons are in place, however train services continue to operate as normal at this time."
South Western Rail have said a small cordon is in place but services are still running.