8 arrested over 'sick' terror attack in London that killed 4, including attacker

— -- Three people were killed, including a police officer, and at least 29 people hospitalized in an attack in London that authorities have declared a terrorist incident. A man believed to be the attacker was also killed, shot by police at the scene.

Eight people were later arrested after overnight raids at six locations including in Birmingham and London, Metropolitan Police acting Deputy Commissioner Mark Rowley said Thursday morning.

Wednesday's attack began when a driver struck pedestrians and three police officers on Westminster Bridge, London's Metropolitan Police said.

Witness Richard Tice told ABC News that he saw injured people on the pavement. According to him, the car jumped the curb, knocking over pedestrians.

The car then crashed into the fence around the Houses of Parliament, and a man armed with a knife attacked an officer who was guarding Parliament, police said.

The suspect, who authorities believe acted alone, was shot and killed by police. In an initial press conference on Wednesday evening, Rowley said the suspect tried to enter Parliament but was stopped "very close to the gate."

The officer who died, identified as 48-year-old Keith Palmer, was not armed, he added. Palmer, a husband and father, served for 15 years with the Metropolitan Police, Rowley said.

The attack -- which occurred on the one-year anniversary of attacks in Brussels that killed 32 people -- was reminiscent of vehicle attacks last year in both Berlin and Nice, France.

Police believe that the attack was "inspired by international terrorism" and that they know the attacker's identity, but Rowley has refused to provide further details. Authorities are also looking at the suspect's possible associates.

As police swarmed the area during Wednesday's chaotic scene, Tom Peck, a British journalist, told ABC News from his office in London that he heard a bang, lots of screaming and then several gunshots.

Authorities said they had received different reports Wednesday of a person in the River Thames, a car that collided with pedestrians and a man armed with a knife.

A seriously injured woman was pulled from the Thames and was among those who received medical treatment, an official with the Port of London Authority told ABC News.

According to The Associated Press, several French teenagers on a school trip, two Romanian tourists, a Chinese citizen and five South Korean visitors were among those injured.

Tobias Ellwood, a member of Parliament, was seen giving first aid to a victim.

Rowley said earlier in the day, "This is a day we plan for but hope it will never happen. Sadly, it is now a reality."

"We will continue to do all we can to protect the people of London," he added.

The Parliament building had been on lockdown after the attack.

The House of Commons and the House of Lords resumed normal operations on Thursday.

"Business must return to normal as quickly as possible," Rowley said Wednesday evening.

U.K. Prime Minister Theresa May described the attack as "appalling" and a "sick and depraved terrorist attack" in a press conference Wednesday evening.

She said the location chosen for the attack was "no accident" and that Britain's threat level will remain at severe, where it has been for some time.

"The terrorists 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.

She continued, "These streets of Westminster, home to the world's oldest Parliament, are ingrained with a spirit of freedom that echoes in some of the furthest corners of the globe. And the values our Parliament represents — democracy, freedom, human rights, the rule of law — command the admiration and respect of free people everywhere."

May offered prayers for the victims and their families and commended the bravery of authorities during the attack "who risk[ed] their lives to keep us safe."

"Once again today, these exceptional men and women ran towards the danger even as they encouraged others to move the other way," she said.

London Mayor Sadiq Khan said in a statement, "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."

"I want to reassure all Londoners and all our visitors not to be alarmed. Our city remains one of the safest in the world," he 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. Londoners will never be cowed by terrorism."

He went on, "My heart goes out to those who have lost loved ones and to everyone who's been affected. Tragically, a Metropolitan Police officer who was doing his duty protecting our city is amongst those who have been killed, and my thoughts are with his family this evening. I want to express my gratitude, on behalf of all Londoners, to the police and emergency services who've shown tremendous bravery in exceptionally difficult circumstances."

Police said a flag with the Metropolitan Police emblem is being flown at half-staff over Scotland Yard in honor of the victims.

Taylor Davis of the U.S. was at the top of the London Eye at the time of the incident.

"We saw a lot of commotion, ambulances, policemen. We kind of thought it was a car accident at first, then a bunch of black detective cars came in a line. That's kind of when we knew it was more serious than that," Davis told ABC News. "It was just surreal how lucky we felt. We felt very safe up there and just being in the right place at the right time."

Anyone with videos or images of the incident is asked to turn them over to police.

President Trump tweeted Wednesday night, "Spoke to U.K. Prime Minister Theresa May today to offer condolences on the terrorist attack in London. She is strong and doing very well."

And U.S. Secretary of State Rex Tillerson said in a statement, "I express my condolences to the victims and their families. 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."

ABC News' Kelley Robinson and Kirit Radia contributed to this report.