Three men have been arrested on suspicion of conspiracy to murder the British soldier who was hacked to death in Woolwich.
Two men aged 24 and 28 were arrested at a residential address in London while a 21-year-old man was arrested in the street, Scotland Yard said in a statement.
Taser electric stun guns were used on the two older suspects.
Police have also been searching four residential addresses in south-east London.
The latest arrests came as Sky News was told Michael Adebowale - one of the men suspected of murdering Drummer Lee Rigby - was detained by police two months ago.
Local shopkeepers say he was part of a group of Muslim activists who had been reported to police several times.
They were told the authorities were aware of him.
A preliminary report into what the intelligence services knew about Adebowale and his fellow suspect Michael Adebolajo is set to be handed over to an inquiry next week.
Sir Malcolm Rifkind, chairman of the parliamentary Intelligence and Security Committee, said he had been briefed by the director general of MI5, Andrew Parker, a couple of days ago.
He added: "What we will be wishing to know is to what extent the intelligence services had any awareness of the two individuals."
Adebowale and Adebolajo are suspected of carrying out the killing in broad daylight last Wednesday.
Drummer Rigby, 25, was hit by a car and then attacked with weapons including a knife and a meat cleaver but the cause of his death was not confirmed by a post-mortem examination.
The two suspects are still under armed guard in hospital after being shot by police. Both are known activists in south-east London.
Meanwhile, MI5 attempted to recruit Adebolajo as an informer six months ago, according to a childhood friend who was arrested shortly after making the claim.
Abu Nusaybah said Adebolajo had rejected approaches from MI5 operatives, but his friend had denied knowing certain individuals the security service was interested in.
Nusaybah was arrested shortly after making the claims in a Newsnight interview, the BBC reported.
He said in his television interview that he thought Adebolajo had undergone a "change" following his detention by security forces on a 2012 trip to Kenya.
He said Adebolajo suggested he had been physically and sexually abused while he was interrogated in the African country, and after this he became withdrawn and "less talkative - he wasn't his usual bubbly self".
Nusaybah said Adebolajo told him that when he returned to the UK he was "followed by MI5" who were "knocking on his door" - a claim that has not been verified by Whitehall.
Nusaybah said: "His wording was, 'They are bugging me - they won't leave me alone.'
"He mentioned initially they wanted to ask him if he knew certain individuals.
"But after him saying that he didn't know these individuals, what he said was they asked him if he would be interested in working for them.
"He was explicit in that he refused to work for them but he did confirm he didn't know the individuals."
Scotland Yard refused to confirm the man arrested at the BBC was Nusaybah, although a spokesman said a 31-year-old man had ben detained at BBC premises in London on suspicion of terrorism offences.
The arrest was made at 9.30pm on Friday by officers from the Metropolitan Police's Counter-Terrorism Command on suspicion of the commission, preparation or instigation of acts of terrorism.
The suspect was taken to a south London police station for questioning. A Scotland Yard spokesman said the case was not directly linked to the killing of Drummer Rigby.
Meanwhile, pictures have been released of Drummer Rigby in 2009 firing a machine gun in Afghanistan, one of the countries where he served.
The family of the serviceman paid a touching tribute when they spoke for the first time on Friday.
The soldier's stepfather, Ian, read a statement from the family: "What can we say about Lee, our hero? We are so so proud of Lee. When Lee was born the family adored him, he was a precious gift given to us.
"Lee's dream growing up was always to join the Army, which he succeeded in doing. He was dedicated and loved his job."
Drummer Rigby's wife Rebecca, the mother of their two-year-old son Jack, said through tears that he was "a devoted father".
A 29-year-old man arrested on suspicion of conspiracy to murder on Thursday has been bailed to return pending further inquiries.
Two women aged 29 and 31 who were also arrested on Thursday on suspicion of conspiracy to murder have both been released without charge.