Theresa May has warned of the "severe" threat facing Britain in the wake of the latest suspected attack on Westminster as new figures showed a huge spike in terrorism investigations.
Downing Street disclosed that the number of live investigations into suspected terrorist plots rose from "more than 500" in March to 676 by the end of June.
A Number 10 spokesman described the current terrorist threat as "one of the starkest we have faced".
New figures also showed that out of 2,029 terrorism-related arrests in Britain over the past seven years, 412 were in the calendar year 2017, the highest number of arrests since records began.
Counter-terrorism police are continuing to question a man who was arrested on Tuesday morning after a Ford Fiesta car swerved across a traffic island outside Parliament and struck three pedestrians before crashing into a barrier outside the House of Lords.
Mrs May has been kept informed of developments at her summer holiday base in Switzerland, and said in a statement: "For the second time in as many years the home of our democracy, which is a potent symbol of our precious values of tolerance and freedom, has witnessed terrible scenes just yards from its door...
“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.”
Mrs May also praised the "formidable courage" of the emergency services, "who were on the scene immediately and in significant numbers.
“Once again, they did not flinch and ran towards a dangerous situation in order to protect the public. The country is hugely grateful for their calmness and professionalism."
She said her thoughts were with "the innocent members of the public who were hurt in this appalling incident".
Senior officials held a meeting of the Cobra emergency committee at 2pm on Tuesday.
Home Secretary Sajid Javid, who was on his way overseas to start a family holiday, returned to London to be briefed on the incident, and is expected to make a statement on Tuesday evening.
A Downing Street spokesman said: "The threat from terrorism is one of the starkest that we have faced. The nature of the terrorist threat is changing and so must our response."
Number 10 said there were no current plans to raise the terrorist threat level, which currently stands at "severe", meaning an attack is highly likely. It is one below the highest threat level of "critical", which would mean an attack was expected imminently.
The Metropolitan Police has said the man who was arrested outside Parliament is not co-operating with the officers interviewing him. The car he was driving was registered in Nottingham.