Suspected plot to disrupt G20 summit follows warnings by police that protesters are planning to bring London to standstill

Five people have been arrested in connection with a suspected plot to use explosives made from fireworks to disrupt the G20 summit.

The three men, aged 25, 19 and 16, and two women, both 20, all live in Plymouth and the surrounding area. They are political activists unaffiliated to any terrorist organisation, and were arrested at addresses in Plymouth. They are being held under terrorism legislation. The explosive devices were made from simple fireworks, police said.

Paul Netherton, Devon and Cornwall assistant chief constable, said imitation handguns and an imitation Kalashnikov, as well as devices made from fireworks, had been seized. The deactivated weapons were "not major" and "probably not even lethal".

Officers said they were investigating the possibility the group planned to attend the G20 summit, which they described as an "obvious line of inquiry".

A police source told the Guardian that initial inquiries indicate the five were "not planning a Guy Fawkes plot". "I think it was more designed to disrupt than injure or kill," the source said.

Scotland Yard has issued warnings in recent days that protesters are planning to bring the capital to a standstill this week in potentially violent protests scheduled to coincide with the arrival of world leaders for the summit. So far, however, protests associated with the summit have been peaceful.

Netherton said the investigation was sparked when the 25-year-old man was arrested for spray-painting on a wall in Plymouth city centre. He would not comment on the nature of the graffiti.

Police searched the man's flat in Plymouth, and found imitation firearms, improvised explosives made of fireworks, and "material related to political ideology".

Asked to clarify the nature of the material, Netherton said: "It's political, it relates to political organisations, it's not extreme but it's a different political view. It leads to motives and things like that."

He confirmed that one of the arrested people was an international student but would not comment on where that suspect studied. The remaining four are all British nationals.

Asked about the potential scale of damage that could be caused by the weapons and devices discovered, Netherton said: "These are imitation firearms, the other devices were made of simple fireworks – they're probably not lethal. But I can't comment. Fireworks do have an explosive component to them but of a minor nature."

Devon and Cornwall police said the five, who were arrested between Friday and Sunday, were being held for offences under the Terrorism Act.

A police statement said: "As the search progressed officers found a number of weapons and suspected imitation weapons as well as material relating to political ideology. Also found were suspicious devices that were seized and submitted for forensic examination."

After investigations on Sunday, a 19-year-old man was arrested. Other addresses were searched during the weekend in the Plymouth area and further searches are being considered. Police said they would be applying for warrants of further detention this afternoon.

Police stressed that inquiries indicated there was no religious element to the investigation, and that those involved were political activists. They are not believed to be part of a national or larger operation.

Assistant chief constable Sharon Taylor said: "I would reassure local people and residents of Devon, Cornwall and the Isles of Scilly that they are not at any risk, and these arrests would appear to be isolated at this time. Those arrested people will remain in custody while further inquiries take place."

A Scotland Yard spokesman said: "The Met has been in liaison with Devon and Cornwall police regarding their ongoing investigation into five people arrested over the weekend. 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."