The Westminster attack triggered a response police hoped would never be needed - but which has been rigorously planned and rehearsed.
Scotland Yard counter-terrorism chief Mark Rowley said the force responded in line with its plans for a "marauding terrorist attack".
Police and security services have been fine-tuning their arrangements for this kind of emergency for a number of years, and fears that London could be targeted have heightened following atrocities in major European cities.
The Metropolitan Police has staged mock assaults to test their response, including an exercise carried out on Sunday.
In the aftermath of the attack, members of the public were told to expect to see more armed and unarmed officers on the streets of the capital.
Personnel from the Met, British Transport Police and City of London will work longer hours and extra shifts, while the military can also be called in should their assistance be required.
A crime scene will remain in place as a painstaking investigation to gather all available evidence is carried out.
Other forces around the UK will be reviewing their arrangements in light of the attack. Essex Police said visible patrols will be increased, including additional armed officers, until further notice.
MPs were kept in the chamber of the House of Commons in the wake of the attack, with the security lockdown lifted nearly five hours later.
Although the Commons and the Lords will sit as normal on Thursday, officials confirmed that tours, educational visits and banqueting in Parliament will be cancelled.
Access to the public gallery will be maintained.
The official threat level for international terrorism has been at severe - indicating that an attack is "highly likely"- for more than two years.
Prime Minister Theresa May confirmed on Wednesday night that the level remains unchanged. Severe is the second highest of five levels, with the highest, critical, indicating that an attack is expected imminently.
The events came a day after both Britain and the US announced new aviation restrictions for some inbound flights.
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