Counter-terrorism police have arrested two men over concerns about a potential extreme rightwing terrorism plot.

The two men, both aged in their 40s, were arrested in south London by officers from Scotland Yard’s counter-terrorism command.

It is believed the arrests followed information being passed to police from a member of the public. It led to a joint operation involving counter-terrorism officers and the UK domestic security service MI5.

It was assessed that the information required further investigation and for arrests to be made, but one strand of inquiries by counter-terrorism investigators was looking at the mental health of those detained.

The first man arrested, aged 40, was held on Monday evening. The Metropolitan police said: “On Monday 29 October, at approximately 22.45hrs, detectives arrested a 40-year-old man on suspicion of the commission, preparation or instigation of acts of terrorism, under section 41 of the Terrorism Act 2000.”

Police have been granted a warrant extending his detention until 5 November.

The second man, aged 47, was detained on Thursday evening. Police said: “Officers arrested a 47-year-old man in south London on suspicion of preparation of terrorist acts, contrary to section 5 of the Terrorism Act 2006, at approximately 18.20hrs on Thursday, 1 November.”

The Met made the arrests public on Friday and said both men were being questioned by detectives at Southwark police station.

Police said searches had been carried out and completed at two addresses in south London linked to the men. A third search at another address was under way.

There has been renewed concern among police and MI5 about extreme rightwing terrorism. The Guardian revealed this week that MI5 will soon take responsibility for fight from the police.