Seven arrests have been made in raids following the Westminster attack that left four dead and 20 others injured, the BBC reported quoting London police as saying.
Acting Deputy Commissioner and Head of Counter Terrorism Mark Rowley said hundreds of detectives have worked through the night, carrying out searches at six addresses.
Those who died were a woman in her 40s, a man in his 50s, PC Keith Palmer and the attacker, he said.
Seven of the injured are still in hospital in a critical condition.
A further 29 had been treated in hospital, Rowley added.
In the attack on Wednesday, a man drove a car along a pavement in Westminster knocking down pedestrians, leaving dozens injured.
He then stabbed a policeman and was shot dead by police in the grounds of Parliament.
In a statement made outside Scotland Yard, Rowley said: "The inquiries in Birmingham, London and other parts of the country are continuing.
"It is still our belief - which continues to be borne out by our investigation - that this attacker acted alone and was inspired by international terrorism.
"To be explicit, at this stage we have no specific information about further threats to the public."
He said he would not name the bridge victims yet, who were "a mix of nationalities", and urged journalists not to publish the attacker's name while searches were continuing.
He said Londoners should expect to see more police officers on the streets, after officers' leave had been cancelled and duty hours extended.
It was initially thought that three members of the public had been killed on Westminster Bridge, but Rowley referred to just two in his statement.