LONDON (AFP) - British police said on Saturday (March 25) that they had released a man arrested for the terror assault on parliament, leaving just one suspect in custody.

A total of 11 people were held on suspicion of preparation of terrorist attacks in the wake of Wednesday's rampage at Westminster, which left four people and the assailant dead.

Only a 58-year-old man arrested on Thursday in Birmingham, the central English city where the assailant Khalid Masood lived, remained in custody on Saturday evening.

One of the 10 others who were released, a 32-year-old woman, is on bail - the others have all been freed with no further action, according to a Metropolitan Police statement.

Masood, a 52-year-old Briton with a history of violent offences, was shot by police on Wednesday after driving into crowds on Westminster Bridge and then stabbing a policeman inside the gates of parliament.

The Islamic State in Iraq and Syria (ISIS) claimed one of its "soldiers" carried out the attack, the worst in Britain since the July 2005 bombings which left 52 people dead.

Counter-terrorism commander Mark Rowley said there was no further threat but police were trying to establish whether Masood acted alone "or if others have encouraged, supported or directed him".