A new arrest has been made in relation to the London terror attack, as officials set their sights on accessing WhatsApp, the heavily-encrypted messaging service that was used by the killer.

The arrest came four days after the lightening assault that unfolded in the shadow of the Houses of Parliament, in which an apparently lone attacker killed four people and wounded 50 before being shot dead by armed police.

A 30-year-old man was detained on Sunday in the central city of Birmingham on suspicion of preparing terrorist acts, London's Metropolitan Police said.

Read more: Women hold hands on Westminster Bridge to remember London attack victims

An officer guards a unit in Birmingham reportedly linked to the London attack. (AAP)

The car rammed into the gates after mowing down pedestrians. (Twitter)

Forensic officials conduct searches in the area around the Houses of Parliament . (AAP)

A dozen people have been arrested since Wednesday's attack by 52-year-old Khalid Masood, who deliberately ran down pedestrians on Westminster Bridge then stabbed a policeman just inside the gates of parliament.

Nine people have been released without charge, while a 58-year-old man remains in custody and a 32-year-old woman has been released on bail.

The arrest came as the government confirmed Masood had used the WhatsApp messaging service, saying it was crucial that the security services be allowed to access the heavily-encrypted app.

Khalid Masood (Image: Scotland Yard)

Media reports said Masood used the Facebook-owned service just minutes before staging his assault, although it was unclear whether he sent any messages or just looked at the app.

Speaking to Sky News, Home Secretary Amber Rudd said it was "completely unacceptable" that police and security services had not been able to crack the heavily-encrypted service.

"You can't have a situation where you have terrorists talking to each other - where this terrorist sent a WhatsApp message - and it can't be accessed," she said.

Police had on Saturday acknowledged they may never know why Masood, a Muslim convert with a violent criminal past, carried out the attack and that he probably acted alone, despite a claim of responsibility by the Islamic State group.

"We must all accept that there is a possibility we will never understand why he did this. That understanding may have died with him," said senior counter-terrorism officer Neil Basu.

Although police believe he acted alone on the day, investigators are still trying to find out whether he was encouraged or directed by others.

"There should be no place for terrorists to hide," Mr Rudd said in a separate interview with the BBC.

“We need to make sure that organisations like WhatsApp - and there are plenty of others like that - don't provide a secret place for terrorists to communicate with each other.”

An injured man is taken away by police after the Westminster Bridge incident. (Getty)

In pictures: London in mourning after Westminster attack View Gallery

WhatsApp said it was "horrified" by the attack and was working with the investigating authorities without saying whether it would change its encryption policy.

"We are horrified at the attack carried out in London earlier this week and are cooperating with law enforcement as they continue their investigations," a spokeswoman told AFP.

Mr Rudd acknowledged that end-to-end encryption was vital to cyber security, to ensure that business, banking and other transactions were safe - but said it must also be accessible.

"It's not incompatible. You can have a system whereby they can build it so that we can have access to it when it is absolutely necessary," she told Sky News.

Police on duty the morning after the attack, (9NEWS)

Tributes left at the scene.

Mr Rudd said she did not yet intend to force the industry's hand with new legislation, but would meet key players on Thursday to discuss this issue, as well as the "constant battle" against extremist videos posted online.

Tech firms and social media players are coming under increasing pressure over extremists using their websites, applications and technology to communicate extremist content.

Last year, US authorities fought a legal battle with tech giant Apple to get it to unlock a smartphone used by one of the shooters in a 2015 attack in San Bernardino, California.

The FBI's own experts ended up breaking into the device.

And Google has faced a boycott by companies whose adverts appear alongside extremist content on its internet platforms, particularly its video-sharing site YouTube.