Eight arrests have been made in raids in London, Birmingham and elsewhere following an attack in Westminster that left four dead, police have said.
Hundreds of detectives worked through the night, carrying out searches at six addresses, Acting Deputy Commissioner Mark Rowley said.
The dead are PC Keith Palmer, Aysha Frade who worked at a London college, a man in his 50s and the attacker.
Seven of the injured are still in hospital in a critical condition.
A further 29 had been treated in hospital, Mr Rowley added.
In the attack on Wednesday afternoon, a man drove a car along a pavement on Westminster Bridge knocking down pedestrians, creating panic and leaving dozens injured.
He then ran towards Parliament where he stabbed PC Palmer who was unarmed. Armed police then shot dead the attacker in the grounds.
Mrs Frade worked at a London sixth form college just a few hundred metres from Westminster Bridge.
Principal at DLD College, Rachel Borland, said she was “highly regarded and loved by our students and by her colleagues”.