A total of 11 people have been arrested as part of this investigation.
A 39-year-old woman was arrested at an address in east London on suspicion of preparation of terrorist acts. She has since been released on bail until a date in late March.
A 21-year-old woman and a 23-year-old man were arrested at an address in Birmingham.
A 26-year-old woman and three men aged 28, 27 and 26 years old, were arrested at a separate address in Birmingham.
All six were arrested on suspicion of preparation of terrorist acts.
A 58-year-old man was also arrested on the morning of Thursday, 23 March at a separate address in Birmingham on suspicion of preparation of terrorist acts.
A 27-year-old man was arrested late on Thursday, 23 March at an address in Birmingham and a 35-year-old man was arrested in the early hours of Friday, 24 March at an address in Manchester.
A 32-year-old woman was arrested on the morning of Friday, 24 March at a location in Manchester.
All three were arrested on suspicion of preparation of terrorist acts
Ten people remain in police custody. Nine were detained under TACT and one was detained under PACE.
The attacker behind the terrorist rampage at the gates of the Houses of Parliament has been named by police as Khalid Masood, 52, who was born in Kent and had previously been convicted of violent offences.
They added that he was known by a number of aliases and had most recently been living in the West Midlands area.
New Scotland Yards Met Police's acting deputy commissioner Mr Rowley said: "Hundreds of detectives have been working through the night and during that time officers have searched six addresses in Birmingham, London and other parts of the country and made eight arrests.
THERESA MAY today revealed a British-born man inspired by Islamist ideology - who had previously been probed in relation to violent extremism.
A minute's silence has been observed at Scotland Yard and the UK Houses of Parliament after the Westminster attacks.
The Met’s head of counter-terrorism, Mark Rowley, said a woman in her mid-40s and a man in his mid-50s were among those killed in the attack as well as PC Keith Palmer, a 48-year-old husband and father.
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