Met Police Commissioner Sir Mark Rowley has called for clarity from the government on extremism as he faces criticism over the handling of pro-Palestinian protests in London.
Five people have been charged after being arrested at the demonstrations in central London on Saturday.
Sir Mark said about 100 people had been arrested at protests since the Hamas attack on Israel three weeks ago.
He added that "many more" arrests are expected in the near future.
Nine people were arrested in central London during a mainly peaceful pro-Palestinian demonstration on Saturday, which was attended by at least 100,000 people calling for a ceasefire in the Israel-Hamas war.
Since Hamas attacked on 7 October, killing more than 1,400 people and taking at least 230 back to Gaza as hostages, Israel has responded with a retaliatory bombing campaign and ground operation.
The Hamas-run health ministry in Gaza has said more than 8,000 people have been killed as a result of Israeli attacks.
In an update on Sunday night, the Met said a 33-year-old man and a 22-year-old woman have been charged with a racially aggravated public order offence, a 51-year-old woman and a 16-year-old boy were charged with public order offences while a 41-year-old man was charged with causing actual bodily harm.
Sir Mark said his officers are limited by legal definitions of extremism and that he would support a review into the definition and how it should be policed.
"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."
He said lawyers from the Crown Prosecution Service are working in the police operations room to help identify offences.
"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.
"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."
Meanwhile, an education minister has said he agreed with the sentiment recently expressed by the deputy mayor of Jerusalem that London has a problem with antisemitism.
Speaking on LBC, Robert Halfon, who is Jewish, warned it is a "fearful time for Jews in the UK", adding there is also a problem of "ever-increasing incidents of antisemitism" in universities.
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