Suspect believed to be Salih Khater, 29, who is of Sudanese origin and from Birmingham

A man has been held on suspicion of terrorism offences after a car was driven into pedestrians and cyclists before crashing outside parliament in Westminster, as Downing Street revealed authorities were working on hundreds of live counter-terrorism investigations.

The suspect, a 29-year-old British national, was arrested at the scene after armed police officers swooped on a silver Ford Fiesta that had crashed into security barriers on St Margaret Street at about 7.40am on Tuesday.

The Guardian understands his name is Salih Khater, who lives in the Sparkbrook area of Birmingham and is of Sudanese origin.

Scotland Yard said the man was not known to the security services and was not cooperating with investigating officers, but owing to the crash appearing to be deliberate and the location it was being treated as a suspected terrorist attack.

The car travelled to London from Birmingham on Monday night and spent the time between around 1.25am and 5.55am in the Soho area, before moving to Westminster and Whitehall at about 6am where it remained before the incident an hour and 40 minutes later, police said.

Two addresses in Birmingham and one in Nottingham were searched on Tuesday. Roger Godsiff, the MP for Birmingham Hall Green, said: “Today’s attack at Westminster was carried out by an individual who is believed to have been living in my constituency in Birmingham.”

Nottinghamshire police said they were supporting counter-terrorism officers as they searched a residential property in the Radford area of the city. The address is thought to be a flat on Peveril Street.

Neighbours told the Press Association that the house, shared by occupants, is home to six Sudanese people. Ibrahim Ahmed, 24, who lives in the same terrace row, said: “It’s Sudanese people who live there.”

The Nottingham Post reported that the car involved in the incident was registered to an address in Nottingham. The current owner of the vehicle bought it on 20 June this year.

Two men and one woman were injured in the crash. One was treated at the scene and two were taken to hospital and eventually discharged.

Theresa May said the threat level to the UK remained at severe, meaning an attack was considered highly likely. The prime minister’s spokesman said the security services and counter-terrorism police were carrying out 676 live investigations at the end of June, up from more than 500 in March.

He said 13 Islamist plots and four plots by far-right extremists had been foiled in the past 18 months.

The home secretary, Sajid Javid, urged people to “keep an open mind” about what had happened. “There are understandably a lot of questions about the incident … The briefing I have received from counter-terrorism police and the security services is that work is ongoing and they are doing everything they can to find out more,” he said.

Facebook Twitter Pinterest Police recover the silver Ford Fiesta. Photograph: Yui Mok/PA

Kirsty Moseley, 31, one of the closest witnesses to the attack, told the Guardian the driver did not shout or say anything as the car collided with as many as 15 cyclists, and he appeared “focused”.

CCTV footage showed the car swerve across a central reservation near Parliament Square and then accelerate down St Margaret Street before crashing into barriers. Pedestrians were seen leaping out of its path.

The scenes had echoes of the Westminster Bridge attack in March 2017, which prompted the extension of steel and concrete security barriers around the Houses of Parliament. Khalid Masood ploughed a car into crowds on the bridge, killing four people, before stabbing and killing an unarmed PC, Keith Palmer.

Another witness to Tuesday’s crash, Ewalina Ochab, said: “The car drove at speed and towards the barriers. I was walking on the other side [of the road]. I heard some noise and someone screamed.”

She said the car did not appear to have a front registration plate. “I turned around and I saw a silver car driving very fast close to the railings, maybe even on the pavement. The person driving did not go out of the vehicle.”

Play Video 1:15 Houses of Parliament: armed police swoop on crashed car – video

Streets around Parliament Square, Millbank and Victoria Tower Gardens were cordoned off and the length of Whitehall was closed to traffic after the incident. Westminster tube station was closed for a time before reopening.

Speaking outside New Scotland Yard, Neil Basu, the head of counter-terrorism policing in the UK, said: “Given that this appears to be a deliberate attack, the method and this being an iconic site, we are treating it as a terrorist incident.”

Westminster security measures face further scrutiny after crash Read more

May, who is on holiday in Italy, was informed of the incident at 8am and was given regular updates by officials. In a statement released by Downing Street, she praised the “formidable courage” and professionalism of the emergency services who “ran towards a dangerous situation in order to protect the public”.

She said: “The threat to the United Kingdom from terrorism remains severe. I would urge the public to remain vigilant but also to come together and carry on as normal, just as they did after the sickening attacks in Manchester and London last year. The twisted aim of the extremists is to use violence and terror to divide us. They will never succeed.”

May’s spokesman said the current terror threat was “one of the starkest that we’ve faced. The nature of the terrorist threat is changing, and so must our response. Between 2010 and 2017 there were 2,029 terror arrests in Great Britain; 412 were made in 2017. That is the highest since records began.”

Play Video 0:21 Westminster car crash: handcuffed man taken away by police – video

Security officials held a Cobra meeting on Tuesday, chaired by the deputy national security adviser, Madeleine Alessandri, and attended by Basu and representatives from the mayor of London’s office.

Downing Street said it was clear the security measures currently in place around the Palace of Westminster had worked effectively and significant numbers of police had been on the scene immediately.

No plans have been made for May to return from her walking holiday in Italy. The chancellor, Philip Hammond, is the most senior minister currently in the UK.

Jeremy Corbyn, the Labour leader, said: “My thoughts are with those hurt and injured outside parliament this morning in what is being treated as a terrorist incident. Our thanks go to our emergency services who responded immediately. Their bravery keeps us safe day in, day out.”

The US president, Donald Trump, tweeted: “Another terrorist attack in London … These animals are crazy and must be dealt with through toughness and strength!”