London was "days away" from a deadly terrorist attack similar to the Sydney siege that saw Islamist Man Haron Monis hold 17 people hostage at a Lindt Chocolate café.
Britain's top police chief Sir Bernard Hogan-Howe said there was "no doubt" there were dangerous people in the country, citing the death of Lee Rigby as an example of the terrorist threat.
Pushed by presenter Nick Ferrari on how close London had been to terrorism spilling onto its streets, Hogan-Howe said a plot had been foiled days before being hatched.
"They have been very close to hurting people badly or killing them," the commissioner said.
Asked if there were extremists like Monis in Britain, Hogan-Howe said: "There will be, there is no doubt. Look at what happened to Lee Rigby.
"There are people out there who are dangerous and that is why we keeping arresting them because we need to stop them hurting people."
The Met Police has prevented five terror plots foiled in the last four months and arrested 35 people.
The most recent terror-related arrests came on Tuesday, when officer's from the Met's Counter Terrorism Command detained two men aged 37 and 61 on suspicion of supporting a banned organisation.
Hogan-Howe added the best way to defend the country from attacks was first of all to rely on good intelligence.