Updated March 25, 2017 16:23:19

British police say they have released without charge six of the 10 people being held in connection with the attack near Britain's Parliament.

The two women and four men were all arrested in the central England city of Birmingham, where attacker Khalid Masood recently lived.

All had been arrested on suspicion of preparing terrorist acts.

Four other people remain in custody.

Police said they believed Masood acted alone, but appealed for information as they tried to establish whether he was directed in any way by friends, family or associates.

Police said Masood was known by a number of names.

Photo: London attacker Khalid Masood was investigated by MI5 over extremism concerns. (AP: Metropolitan Police)

Born Adrian Elms in the county of Kent, he later took his stepfather's name and was known as Adrian Ajao by high school.

Police said he had begun to earn a reputation for violence during his 20s and after spending time in three different prisons, he had visited Saudi Arabia, converting to Islam at least seven years ago.

Leaders gather to mourn victims and call for solidarity

Britain's most senior faith leaders gathered near the country's Parliament to mourn the victims of the attack and to express their condolences.

Sheikh Mohammed Al-Hilli, representing Shia Muslims, and Sunni Sheikh Khalifa Ezzat, head Imam at the London Central Mosque, stood alongside the Archbishop of Canterbury, Justin Welby, chief rabbi Ephriam Mirvis and Catholic Cardinal Vincent Nichols at the event.

"We utterly condemn this terrorist act," Sheiokh Al-Hilli said.

Photo: Passers-by look at floral tributes left for the victims of the Westminster attack. (ABC News: Lisa Millar)

Meanwhile, London Mayor Sadiq Khan joined other Londoners in leaving floral tributes to honour Constable Keith Palmer, the police officer killed by Masood.

Mr Khan's note and flowers were placed outside of New Scotland Yard, the headquarters of the Metropolitan Police Force.

"You will always be in our hearts. Londoners will never forget the innocent people who lost their lives," Mr Khan said in his note.

Prime Minister Malcolm Turnbull has spoken directly to his British counterpart Theresa May about the terrorist attack.

He expressed his condolences and said Australia stood in solidarity with Britain.

MPs who assisted after attack receive royal honour

Meanwhile, a member of parliament who battled to save the life of Constable Palmer has been honoured with an appointment as an adviser to Queen Elizabeth II.

British Prime Minister Theresa May's office said Tobias Ellwood had been named to the Privy Council, a committee of senior MPs, judges and others that advises the Queen.

Security Minister Ben Wallace, who helped coordinate the Government response to the attack, has also been named to the Council.

Mr Ellwood, a former army officer whose brother died in the 2002 Bali bombing, gave mouth-to-mouth-resuscitation to Constable Palmer after he was stabbed by Masood.

ABC/AP

Topics: terrorism, unrest-conflict-and-war, police, united-kingdom

First posted March 25, 2017 13:32:20