A suspected terrorist plowed a car into pedestrians on London's Westminster Bridge on Wednesday afternoon, then fatally stabbed a police officer before being shot.
In a Wednesday evening press briefing, Mark Rowley, assistant commissioner of the Metropolitan Police, said that the attack left four people dead: two members of the public, the stabbed officer and the attacker. This figure was revised down from the earlier five.
Police said about 29 other people were being treated in hospital, seven of who were is a severe condition.
Local law enforcement currently believe that the dead attacker is the only suspect, but Rowley declined to comment on the assailant's identity. The police's working assumption is that the attacker was inspired by international Islamist terrorism.
Londoners can expect to see heavier police presence, both armed and unarmed, throughout the city for the next few days, Rowley said.
The Parliament was quickly placed on lockdown in accordance with protocol for what authorities call a marauding terrorist attack. Prime Minister Theresa May and Queen Elizabeth II were reported safe.
While the lockdown has been lifted, much of the area remains cordoned off as police continue with their investigation.
On Wednesday evening, May gave a stirring speech, in which she applauded the heroism demonstrated by the police. The location of the attack, steps away from the seat of British government, was "no accident," May said.
"The terrorist chose to strike at the heart of our capital city, where people of all nationalities, religions and cultures come together to celebrate the values of liberty, democracy and freedom of speech," she said.
"But let me make it clear today, as I have had cause to do before, any attempt to defeat those values through violence and terror is doomed to failure."
Londoners and tourists will wake up on Thursday and go about their lives as normal, May said.
Mayor Sadiq Khan made similar remarks earlier, saying "Londoners will never be cowed by terrorism."
"I want to reassure all Londoners and all our visitors not to be alarmed — our city remains one of the safest in the world," Khan said. "London is the greatest city in the world and we stand together in the face of those who seek to harm us and destroy our way of life. We always have, and we always will."
Police confirmed that a full counterterrorism operation is underway and that they are increasing law enforcement presence around the city.
The Port of London Authority said in a statement that a woman has been pulled from the River Thames. The agency said the woman was alive but has sustained very serious injuries.
The incident at Parliament occurred on the first anniversary of the terrorist bombings in Brussels that killed 32 people.
France's Foreign Ministry said in a statement that three French high school students were in serious condition after being run over on Westminster Bridge.
French Prime Minister Bernard Cazeneuve tweeted in French, "Solidarity with our British friends terribly stricken, full support to the injured French students, their families and their comrades."
The U.K. government said both the House of Commons and House of Lords will sit their scheduled sessions on Thursday.
President Donald Trump has spoken with May, White House spokesman Sean Spicer said during his daily press briefing. The president pledged the "full cooperation and support of the United States government in responding to the attack and bringing those responsible to justice," according to the White House readout of the call.
Secretary of State Rex Tillerson expressed his condolences in a statement.
"The American people send their thoughts and prayers to the people of the United Kingdom. We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference," Tillerson said in a Wednesday statement.
Before the incident, the United Kingdom was on its second-highest level of alert — "severe" — meaning that was terrorist attack was considered highly likely.
Brian Dillon, former head of the firearms command of London's Metropolitan Police, told NBC News that, "This is exactly the sort of thing that police feared might happen, feeding into the high threat assessment for the U.K."
"This is one of a number of concerns that were feared, with a vehicle and weapons being used," said Dillon, who now runs the counterterrorism consultancy Rubicon Resilience.
A spokesman for the U.S. State Department said, "We are closely following the concerning situation outside the U.K. Parliament."
"Our hearts go out to those affected. We stand ready to assist in any way the U.K. authorities would find helpful. The safety and security of U.S. citizens overseas is one of our highest priorities," said the spokesman Mark Toner.
"Our embassy in London is monitoring the situation closely. We stand ready to provide all possible consular assistance should we become aware of any affected U.S. citizens," Toner said. "We urge U.S. citizens in London to contact family members and loved ones to notify them that you are safe, to avoid the area and monitor local news or follow @metpoliceuk for updates."
Clarification: This story has been updated since first published following revised casualty and fatality figures from the Metropolitan Police.
— NBC News' Alexander Smith and CNBC's Willem Marx in London contributed reporting.