POLICE say they have seized a deactivated Kalashnikov rifle, imitation handguns, knives and a number of devices "made from fireworks" as part of a terrorism investigation.

Two men aged 25 and 19, a 16-year-old schoolboy and a 20-year-old woman were this morning still being held at Launceston police station under the Terrorism Act. Another woman, also aged 20, was released from police custody late last night following a court hearing. All five appeared at Plymouth Magistrates Court yesterday afternoon after police applied for warrants of further detention. The hearing was presided over by a London judge who specialises in terrorist matters.

The investigation came about after a sharp-eyed police officer stopped a 25-year-old man – believed to be Andrew Sprague – on Friday night daubing anti-fascist graffiti along the North Street subway under Exeter Street, just a couple of hundred yards from Charles Cross police station.

It is understood the man had sprayed the word "Antifa" – a militant anti-fascist organisation with international links. Their website claims their primary "enemy" is the British National Party, who they say have in recent years "done their utmost to hide their fascist politics beneath a thin veneer of respectability".

Their mission statement goes on to recognise violence as a necessary tactic to counter fascist groups.

The officer and a colleague carried out a search of an address in Radnor Place, Greenbank and found three other people at the premises who were initially arrested for drugs offences.

However, further searches of the property quickly changed the nature of the investigation as police found the deactivated AK47 assault rifle, imitation handguns and knives.

They also found devices which police have admitted were made from fireworks.

In addition, police said they found "literature relating to political ideology" at the address and computers have also been seized for examination.

Police said that further searches at other addresses in the city resulted in a 19-year-old being arrested on Sunday under the Terrorism Act.

Assistant Chief Constable Paul Netherton, speaking at a packed press conference yesterday afternoon, said the warrant for further detention would allow investigators more time to search a number of different locations around the city and have items seized submitted for forensic examination.

He said: "We are very keen to stress that these arrests do not relate to religious extremism and in particular the Nicky Reilly case, nor to the Islamic faith."

He said the residents of the area were not at risk and the arrests were an "isolated incident".

Asst Chief Constable Netherton admitted that four of those arrested were British, but one was an "international student", who some sources have claimed is of Greek origin. While some media outlets have claimed the arrests were connected with the expected anti-globalisation protests during the forthcoming G20 summit in London, Asst Chief Constable Netherton said he "could not confirm [the connection] at this time".

He added: "Quite obviously we're carrying out interviews with them at this time and we will be asking those kind of questions."

He also stressed that the weapons found were "not major" and "probably not even lethal" while the political literature did not relate to any extremist organisation.

In addition, he said Devon and Cornwall Constabulary was "working with other forces around the country about what we've found", which suggests the Metropolitan Police – who are co-ordinating the response to the G20 protestors – were being kept informed of developments.

The terraced property in Radnor Place is in the heart of the student-land, with halls of residents and student flats close by. Officers were still at the address last night carrying out further searches of the building.

A spokesman for Plymouth City Council confirmed its clean-up staff had been sent to paint over the graffiti, which is still partially visible.