The 34-year-old was held in Chelsea, south-west London, on Monday.
A third man has been arrested on suspicion of arson over attacks on two properties and a car linked to Sir Keir Starmer.
The 34-year-old was held in Chelsea, south-west London, on suspicion of conspiracy to commit arson with intent to endanger life on Monday, the Metropolitan Police said.
His arrest follows that of a 26-year-old man, who was detained on suspicion of the same offence on Saturday at Luton Airport.
Roman Lavrynovych, a 21-year-old Ukrainian man, has already been charged with three counts of arson with intent to endanger life in connection with the fires.
Lavrynovych, of Sydenham, south-east London, denied arson in a police interview.
He appeared in court on Friday and was remanded in custody until a further hearing at the Old Bailey scheduled for June 6.
