LONDON -- A knife-wielding man went on a deadly rampage at the heart of Britain’s seat of power Wednesday, mowing down pedestrians on London’s Westminster Bridge before stabbing an armed police officer to death inside the gates of Parliament.

Five people were killed, including the attacker, and at least 40 people were injured, police said.

Lawmakers, lords, staff and visitors were locked down as the man was shot by police within the perimeter of Parliament and just yards from entrances to the building itself. He died, as did two pedestrians on the bridge, and the police officer.

Metropolitan Police counterterrorism chief Mark Rowley said police believed there was only one attacker, “but it would be foolish to be overconfident early on.”

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The threat level for international terrorism in the U.K. was already listed at severe, meaning an attack is “highly likely.”

Speaking outside 10 Downing St. on Wednesday night, Prime Minister Theresa May said that level would not change. She said attempts to defeat British values of democracy and freedom through terrorism would fail.

“Tomorrow morning, Parliament will meet as normal,” she said.

She said Londoners and visitors “will all move forward together, never giving in to terror and never allowing the voices of hate and evil to drive us apart.”

In a statement, London Mayor Sadiq Khan said the attack is being considered a “terrorist attack.”

“Londoners should be aware that there will be additional armed and unarmed police officers on our streets from tonight in order to keep Londoners and all those visiting our city safe,” Khan said in a statement. “I want to reassure all Londoners and all our visitors not to be alarmed - our city remains one of the safest in the world.”

The attack began on Westminster Bridge in the shadow of Big Ben, an iconic landmark and major tourist attraction in London, CBS News foreign correspondent Jonathan Vigliotti reports. Police say the suspect used his vehicle to plow onto the sidewalk of the bridge, injuring people who were there. The suspect then took the vehicle and smashed into the gate of Parliament, where police say he got out and stabbed a police officer before two other officers shot him.

Hundreds of people were inside Parliament and were forced into lockdown. Among them was Prime Minister Theresa May, who was seen in a video a short time later being safely escorted out.

The suspect’s identity and motive remain unclear, Vigliotti reports.

London Police Commander B.J. Harrington said a full counter-terrorism investigation was underway. There was no immediate claim of responsibility.

The attack comes on the one year anniversary of a double suicide bombing in the Belgian capital, Brussels, that left more than 30 people dead. The Islamic State of Iraq and Syria (ISIS) was behind that attack.

In New York, police have tightened security at British diplomatic posts and at other locations.

London’s Metropolitan Police said they were called at approximately 2:40 p.m. local time to reports of the incident on Westminster Bridge, which is located just outside the Palaces of Westminster, home of the Parliament.

Photos posted on social media showed a dark SUV had come to rest with its front end smashed into a fence around the perimeter of the Houses of Parliament compound.

Terror attack outside U.K. Parliament. 2 men down on road. Man with knife/machete got into Parliament and stabbed policeman. He's been shot pic.twitter.com/aYNktTiF4d — James West (@westicles69) March 22, 2017

Rick Longley told the Press Association that he heard a bang and saw a car plow into pedestrians and come to a crashing stop. Images from the scene showed pedestrians sprawled on the ground, with blood streaming from a woman surrounded by a scattering of postcards.

“They were just laying there and then the whole crowd just surged around the corner by the gates just opposite Big Ben,” he said. “A guy came past my right shoulder with a big knife and just started plunging it into the policeman. I have never seen anything like that. I just can’t believe what I just saw.”

It all unfolded within sight of the London Eye, a large Ferris wheel with pods that have views over the capital. It stopped rotating and footage showed the pods full as viewers watched police and medical crews on the bridge, which has at its north end Big Ben and Parliament, two iconic symbols.

“The whole length of the bridge there were people on the ground,” Richard Tice, a witness, told Sky News. The London Ambulance Service said it had treated at least 10 people on the bridge.

Colleen Anderson of St. Thomas’ Hospital said some people had minor injuries and that others were “catastrophic.” British port officials said they pulled a woman from the River Thames. She was injured but alive after the attack.

Romain Nadal, a spokesman for the French Foreign Ministry, said three students from Saint-Joseph high school in Concarneau who were on a school trip were among the injured. Romania’s foreign ministry says two Romanians were wounded.

None of the victims were American citizens, CBS News has learned.

After leaving a trail of destruction on the bridge, the attacker managed to get through tall iron gates and into Parliament’s New Palace Yard, a cobbled courtyard in the shadow of the Big Ben clock tower. Just yards to the right is the entrance to 1,000-year-old Westminster Hall, the oldest part of the parliamentary complex, busy with visitors and school groups. Beyond that, a corridor leads to the building’s Central Lobby, flanked by House of Commons and House of Lords chambers.

One British lawmaker was hailed by some as a hero. Conservative parliamentarian Tobias Ellwood, whose brother was killed in the Bali terror attack in 2002, performed mouth-to-mouth resuscitation on the police officer who was stabbed and later died. About 10 yards away from the police officer was the attacker who was shot dead by police after scaling the security wall toward the Parliament’s grounds.

Ellwood, who served in the British military and served in Bosnia, Northern Ireland, Kuwait and Cyprus, applied pressure to the police officer’s multiple lacerations. Photographs showed Ellwood’s bloodied hands and face from the police officer’s wounds while the alleged attacker was seen nearby. Ellwood has been an undersecretary at the Foreign Office since 2014, covering the Middle East and Africa.

President Donald Trump spoke with U.K. Prime Minister Theresa May to offer his condolences on what the White House called a “terror attack,” according to a White House statement. The statement also said Mr. Trump offered his “praise for the effective response of security forces and first responders.”

“He pledged the full cooperation and support of the United States Government in responding to the attack and bringing those responsible to justice,” the statement said.

The U.S. Homeland Security Department said the security posture in the U.S. has not changed in the wake of the attack.

U.S. Secretary of State Rex Tillerson expressed his condolences to the victims and their families on behalf of the United States.

“The American people send their thoughts and prayers to the people of the United Kingdom,” he said in a statement. “We condemn these horrific acts of violence, and whether they were carried out by troubled individuals or by terrorists, the victims know no difference.”

The U.S. State Department said it was closely monitoring the incident outside London’s Parliament and urged Americans in London to avoid the area.

Spokesman Mark Toner said Wednesday: “We stand ready to assist in any way the U.K. authorities would find helpful.”

He added that the U.S. Embassy in London is closely following the news and stands ready to help any affected Americans.

He said: “Our hearts go out to those affected.”

Other world leaders spoke out after the attack. Among them were French President Francois Hollande and German Chancellor Angela Merkel, who both expressed their support and solidarity with Britain.

Hollande said that countries “must bring all the conditions to answer these attacks” and that “it is clear that it is at the European level, and even beyond that, that we must organize ourselves.” Merkel said in a statement Wednesday that she learned “with sorrow” of Wednesday’s incident and her thoughts were “with our British friends and all of the people of London,” in particular those who were injured.

Spanish Prime Minister Mariano Rajoy condemned the attack and offered condolences to the British people in a telegram sent to Theresa May. Russian Foreign Ministry spokeswoman Maria Zakharova expressed sympathy to those injured and condolences to the relatives of those who died in the incident at Britain’s parliament, and underlined the need for global cooperation in the fight against terrorism. Israel also expressed solidarity with the victims.

“Israel expresses its deep shock at the terror attack in London today and its solidarity with the victims and with the people and government of Great Britain. Terror is terror wherever it occurs and we will fight it relentlessly,” Israel’s Deputy Foreign Minister Tzipi Hotovely said in a statement.

British security thwarted some 13 terror plots over the past four years, but the U.K. has largely been spared major international terror attacks such as the ones seen in Belgium and France.

Last year, a far-right supporter shot and killed British lawmaker Jo Cox, who had campaigned for the U.K. to remain in the European Union. Prior to that, an attacker stabbed three people at a train station in east London in response to the Royal Air Force’s bombing of the Islamic State group in Syria.

The most gruesome recent attack occurred in 2013 when two Muslim converts of Nigerian descent attacked Lee Rigby, a British Army soldier who was walking down the street. The men ran Rigby down with their vehicle and then used a cleaver to hack him to death as bystanders watched in horror.

The worst peace time attack on Britain this century was on July 7, 2005, when four al Qaeda-inspired bombers blew themselves up on three subway trains and a bus in London, killing 52. Three of the bombers were British-born, all of Pakistani descent; the other emigrated from Jamaica.

On Wednesday, the Italian interior ministry said the nation’s top security and intelligence officials will huddle in Rome on Thursday for “an evaluation of the terrorist threat” after the attacks in London. The ministry said minister Marco Minniti convened the Committee of Strategic Anti-terrorism Analyses following “the tragic facts in London.”

Italian security was already on high alert for a European Union summit bringing EU nation leaders to Rome Friday for a meeting with Pope Francis at the Vatican, and a ceremony in the Italian capital Saturday.