Counter-terror police are investigating three explosive packages found at Heathrow Airport, London City Airport and Waterloo station.
The “small improvised explosive packages” were found in A4 postal bags, the Metropolitan Police said.
The force’s Counter Terrorism Command is treating it as a “linked series” and “keeping an open mind” about motives.
Irish police are assisting the Met as the Heathrow and Waterloo packages had Republic of Ireland stamps
Analysis: BBC News Home Affairs Correspondent Daniel Sandford
Fortunately these packages only appear to be designed to start a very small fire – the one that went off just melted part of its own plastic envelope, and the other two were not opened.
But there is sufficient concern about today’s incidents for them to be investigated as a linked series by Scotland Yard’s Counter Terrorism Command.
This means the full weight of resources and expertise of one of the world’s most experienced counter terrorism teams will be trying to get to the bottom of who sent the packages and why.
The motive is unclear. It could be anything from Irish republicanism to a grievance against transport companies. Other possibilities include someone with strong opinions about Brexit or someone with mental health problems.
The devices do not seem to be capable of causing serious injury, so they were probably intended to have a nuisance effect and to generate publicity, which they have successfully done.
Police will be hoping the series is now over.
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