Detectives removed forensic evidence last night from the flat of a man being held under counter-terrorism powers.

The suspect, named by sources as Andrew Sprague, reportedly in his mid-twenties, was one of five people to be arrested under counter-terrorism powers in Plymouth, Devon, in the past three days.

Police refused to confirm or deny the arrest was connected to a plot to disrupt the G20 summit.

Officers dressed in blue overalls removed clear plastic bags containing documents from the flat in Radnor Place last night.

Police swooped on homes in the city after officers uncovered a cache of imitation firearms, makeshift explosive devices and "political literature".

Three men and two women have been held under the Terrorism Act over the past three days, Devon and Cornwall Police said.

Sources earlier said the five suspects were being questioned over claims that they planned to target the meeting of the world's most powerful political leaders in London later this week.

Mother and daughter Sandra and Sophie Cramb said Sprague's flat, three floors above their own home, was raided by police on Friday evening.

Mrs Cramb, 46, said the tenant, a man in his mid-20s, had lived there for roughly two months and frequently had friends dressed in a "goth" style with "chains and black clothing" to visit.

She said: "We've not been told anything about what's been going on up there.

"The police told us it was something minor.

"It's scary to think this sort of thing could be going on right above us."

At a press conference at Crownhill Police Station in Plymouth yesterday afternoon, Assistant Chief Constable Paul Netherton said the investigation was sparked when a 25-year-old man was arrested for spray-painting on a wall in Plymouth city centre – but would not comment on the nature of the graffiti.

Police said during the search of the man's flat they found a number of imitation firearms, improvised explosives made from fireworks, and material related to political ideology.

Mr Netherton 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.

Police said the three men, aged 25, 19 and 16, and the two women, both aged 20, all lived in Plymouth or surrounding area and were arrested between Friday and Sunday.

Mr Netherton said: "Devon and Cornwall Constabulary are currently detaining five people – three men and two women – for offences under the Terrorism Act."

"Police will be applying for warrants and further detention orders under the Terrorism Act in order to continue to search various properties in the Plymouth area and to interview the five suspects."

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.