Two more arrests have been made by Counter Terrorism Police investigating an alleged conspiracy to commit arson at a site connected to the Jewish community, the Metropolitan Police said.
A total of nine people have now been arrested over the past three days as part of an ongoing investigation.
The latest arrests are a 19-year-old man and a 26-year-old man who were arrested in Watford on Tuesday.
Both men remain in custody, police said.
Those arrested are suspected of being involved in planning an arson attack at an unknown venue related to the Jewish community.
Seven others who were arrested for conspiracy to commit arson remain under investigation and have been released on bail, with strict conditions.
These include three men arrested in Harpenden on April 19 aged, 24, 26 and 26.
A 25-year-old man was also arrested in Stevenage.
A 26-year-old man and two women aged 50 and 59 were detained in a car near Birmingham on Monday in connection with the alleged conspiracy.
Senior National Co-ordinator of Counter Terrorism Policing, Deputy Assistant Commissioner Vicki Evans, 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.
“My message to anyone even considering getting involved in this type of activity is this, the stakes are high and it is absolutely not worth the risk.”