Officers from Special Branch arrested Del O'Connor after he arrived back in Britain from the United States. He was released on bail.
Mr O'Connor, 39, is the alleged leader of the White Wolves, a renegade faction of extreme right-wing group Combat 18, which has allegedly called on its supporters to launch a race war.
He was arrested as part of an ongoing investigation by Scotland Yard into the distribution of race-hate material. On 5 March, four people were arrested and released on bail as part of the operation.
A Scotland Yard spokeswoman said: "A 39-year-old man was arrested in connection with the distribution of race-hate material.
"It is part of an ongoing investigation into the extremist group known as Combat 18."
As well as being questioned about the operation in March, Mr O'Connor is understood to have been quizzed on the race-hate material distributed during the three nail-bomb attacks in London's Brixton, Brick Lane and Soho in April.
A stencilled message from the "Command Council of the White Wolves" was received by the Eastern Eye newspaper two days before the Brixton bombing on 17 April.
Another message sent after the blast said: "C18 did not carry out the Brixton bombing. We, the White Wolves, did."
It was not clear why Mr O'Connor returned from the United States but it was thought to be linked to a visa problem.
It was not known if American officials had tipped off British police.
"Why he left the States is a matter for the US authorities," the police spokeswoman added.
t David Copeland, from Cove, Hampshire, has been charged with murder in connection with the London nail bombings. He is awaiting trial.Reuse content