Five people arrested under counter-terrorism powers may have planned to disrupt the G20 Summit, sources said today.
Police swooped on a series of homes in Plymouth after officers uncovered a cache of weapons, suspicious devices and suspected extremist material during a search.
The three men and two women were held under the Terrorism Act over the last three days, Devon and Cornwall Police said.
They were being questioned over claims they planned to target the meeting of the world's most powerful political leaders in London later this week.
Scotland Yard officers are liaising with investigators in south west England over the suspected plot.
One source said the group are political activists who may have aspired to disrupt the G20 summit but not to injure or kill.
It is understood search teams have found a small number of suspicious devices that indicate any action would have been on a small scale.
Officers in Plymouth uncovered a cache of weapons, suspicious devices and suspected extremist material after arresting a man on suspicion of criminal damage.
The devices have been sent for further forensic examination and police have not ruled out further searches as the net widens.
Police said the investigation is not linked to any particular religious group and centres around "political activity involving British nationals".
Scotland Yard has issued warnings that protesters are planning to bring the City of London to a standstill with high-profile protests this week.
The G20 summit, to be held at the Excel arena in London's Docklands, has been described by senior officers as the capital's most challenging police operation for a decade.
Several thousand officers will be on duty when the leaders of the world's 20 richest countries, including US President Barack Obama, descend on the capital.
A Metropolitan Police spokesman said: "We would stress that the investigation is at a very early stage and that speculation regarding the capability, intention or motivation of those arrested is unhelpful.
"At the current time we have no information to suggest a change to the threat picture facing either the demonstrations or G20.
"The Metropolitan Police has said that we are seeing an unprecedented level of activity amongst protest groups not seen since the late 1990s, involving some individuals we have not seen on the protest circuit for some time.
"However, while these individuals are talking about what they would like to happen, we are unsure of how much of this is achievable or simply aspiration.
"The Metropolitan Police has a duty to have a policing operation in place to be able to respond to any eventuality. Not to do so would be to neglect our duty to Londoners.
"The Metropolitan Police is very experienced in policing protests and public order situations and always seeks to facilitate lawful demonstration."