Metropolitan Police Commissioner Sir Mark Rowley said his officers have been policing pro-Palestine protest "robustly" but added there can only be prosecutions when the law is broken.
He told Trevor Phillips on Sunday on Sky News: "We will robustly enforce up to the line of the law.
"We're going to be absolutely ruthless and we have been and you'll see many more arrests over the next week or so."
Sir Mark's comments come after police clashed with pro-Palestine protesters on Saturday.
Although the demonstrations were largely peaceful, the Met said they arrested nine people - seven for public order offences and two for assaulting police officers.
Overall, Sir Mark said the Met has made "100 arrests in these events in recent weeks" but they are still appealing to identify a number of others.
Of those seen wearing images of Hamas paragliders, Sir Mark said they had not been arrested yet but will be once identified.
"There are many appeals out there to identify a number of people," he said.
The Met chief also said his officers were working with CPS lawyers in the operations room to identify potential offences.
He told Sir Trevor Phillips on Sky News: "We've got these big protests and some of what goes on there, people do find it upsetting and distasteful and sometimes people give an instinctive view that must not be legal.
"But there's no point arresting hundreds of people if it's not prosecutable, that's just inflaming things."
Amid questions over the definition of extremism, Sir Mark said he would support a review into the legal definition and how it should be policed.
He added: "There is scope to be much sharper in how we deal with extremism within this country.
"The law was never designed to deal with extremism, there's a lot to do with terrorism and hate crime but we don't have a body of law that deals with extremism and that is creating a gap."
