Theresa May warned today there are potentially thousands of people at risk of being radicalised in the UK as she indicated plans for a fresh crackdown on extremists groups.
The Home Secretary said around 500 police and security officers were working on the "horrific murder" of soldier Lee Rigby.
Mrs May said "all the indications" pointed to a lone wolf style incident rather than a wider planned operation.
Asked if she believed there were thousands at risk of radicalisation in the UK, she replied "potentially".
Mrs May suggested a lower limit for imposing banning orders on extremist groups could be introduced.
She told the BBC's Andrew Marr Show: "We do need to look at the powers, we do need to look at the laws.
"We do need to look, for example, at the question of whether perhaps we need to have banning orders to ban organisations that don't meet the threshold for proscription."
Tighter rules could also be imposed on internet service providers, the Home Secretary suggested.
She added: "One of the issues we need to look at is whether we have got the right processes, the right rules in place in relation to what is being beamed into people's homes."
Mrs May defended the "excellent" work of the security and intelligence agencies in the face of claims mistakes were made in the handling of the two suspects who were known to MI5.
A childhood friend of one of the suspects has also claimed that the suspect was approached by MI5 six months ago and asked if he would work for the security service.
Mrs May refused to be drawn on the details of the case but said said the security services worked in a "variety of ways".
"They will approach individuals from time to time," she added.
Detectives have made three further arrests in connection with the brutal killing. Two men, aged 24 and 28, were detained at an address in south-east London yesterday, while a 21-year-old man was arrested in the street in Charlton Lane, Greenwich, all on suspicion of conspiracy to commit murder.
Police from the Counter Terrorism Command, supported by specialist firearms officers, used Tasers to detain two of them, the 21 and 28-year-old. They did not need hospital treatment.
The arrests came as Downing Street confirmed the launch of a new terror task force to crack down on extremism.
The group, comprising Cabinet ministers and top police and security service officials, will focus on radical preachers who seek out potential recruits in prisons, schools, colleges and mosques.
The Prime Minister has also announced that the parliamentary Intelligence and Security Committee (ISC) will carry out an investigation following the disclosure that the two men suspected of murdering Drummer Rigby - identified as Michael Adebolajo, 28, and Michael Oluwatobi Adebowale, 22 - were known to MI5.
Police last night refused to comment on reports that Adebowale was arrested two months ago.
Authorities in France are also investigating whether the murder of Drummer Rigby was linked to an attack on a French soldier, who was stabbed in the neck in a busy shopping area near Paris yesterday.
The latest arrests in London occurred between 6pm and 6.30pm, and the three men are being held at a south London police station, Scotland Yard said.
Officers were last night searching four addresses in south-east London.
Searches have also taken place at three other addresses in south London, one in east London, one in north London and one in Saxilby, Lincolnshire, the former home of Adebolajo.
A 29-year-old man arrested on suspicion of conspiracy to murder has now been released on bail, police said last night.
Two women aged 29 and 31 have been released without charge after they were held on Thursday on suspicion of conspiracy to murder, and a number of other people not directly involved with the attack have been charged over malicious comments made on social networking sites.
Adebolajo and Adebowale remain in a stable condition in hospital after being shot by police when they charged towards armed officers in Woolwich on Wednesday.
Drummer Rigby was hit by a car and then attacked with weapons including a knife and a meat cleaver.
The cause of his death has not yet been confirmed by a post-mortem examination.
The young father's murder has provoked a backlash of anger across the country, with mosques being attacked, widespread racial abuse and comments on social media and a large increase in anti-Muslim incidents.