Two more people have been arrested on suspicion of being involved in planning an arson attack at a venue related to the Jewish community, the Metropolitan Police said.
The two men, aged 19 and 26, were taken into custody in Watford, where they remain.
A total of nine people have been arrested over the past three days as part of a proactive investigation into an alleged conspiracy to commit arson, the force said in a statement.
Three men, aged 24, 25 and 26, were detained in Harpenden shortly after 9pm on Sunday, 19 April. They were taken into custody before being released on bail.
The day after, on Monday, a 25-year-old man was arrested in Stevenage. A 26-year-old man, and two women, aged 50 and 59, were also arrested in a car near Birmingham. They have since been released on bail.
On Tuesday, another man, aged 39, was arrested at an address in Ealing in relation to an investigation following the discovery of jars of a non-hazardous substance in Kensington Gardens on Friday, 17 April. He can be kept in custody until 28 April due to the police obtaining a warrant of further detention.
Deputy Assistant Commissioner Vicki Evans, the Senior National Co-ordinator of Counter Terrorism Policing, said: "Our investigations continue at pace, and we are continuing to identify and arrest suspects who we believe were involved, or planning, the recent arson attacks in north west London.
"We've now had the first conviction in relation to this recent spate of arson attacks and seven others have been charged. We remain determined to identify, arrest and charge all those who have been involved in the recent criminal activity."
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