Those who have been arrested under the Terrorism Act live in Plymouth, Devon and the surrounding area. They are being held at a police station in Cornwall.
According to The Telegraph, police found a cache of imitation firearms, knives and a number of "suspicious devices", besides a large amount of political material that is thought to relate to anti-capitalist protests planned to disrupt the meeting of world leaders this week.
A police spokesman said there was no suggestion of Islamic extremists being involved and added: "Those arrested are not believed, at this stage, to be part of a national or larger operation."
Officers found a range of imitation and deactivated firearms from handguns to long barrelled rifles and a Kalashnikov-style assault rifle, police said. They also found 'suspicious devices,' described as 'improvised explosives made of fireworks,' that were seized and submitted for forensic examination.
The discoveries followed a number of raids in Plymouth after a 25-year-old man was caught spraying graffiti slogans in the city centre on Friday, Mar 27 night.
One of those arrested was said to be an international student and the other four are British nationals.
Police were granted more time to question them on Monday, Mar 30 afternoon.
The G-20 summit, to be held at the ExCeL arena in London's Docklands on Thursday, Apr 2 has been described by Scotland Yard as the capital's most challenging police operation in a decade.
Police fear the rallies will be hijacked by anarchists intent on sparking violence and riots, with thousands of hardline protesters expected to fly in from abroad.