The arrest of a serving Metropolitan Police officer relates to the outlawed neo-Nazi group National Action, it is understood.
Counter-terrorism officers arrested the 21-year-old police constable in north London.
He is being held on suspicion of membership of a proscribed organisation linked to right-wing terrorism, the Met said.
Officers are searching the address where he was arrested.
A mandatory referral to the Independent Office of Police Conduct has been made, the force said.
"Whilst the investigation remains ongoing, at this time there is nothing to suggest there is any threat to wider public safety in relation to this matter," the force added.