On Wednesday 22 March in the afternoon, there was a serious incident outside Parliament in London, which police have described as a terrorist attack.
Six people were killed, including the attacker who was shot dead by armed police.
On Thursday morning, the UK prime minister confirmed in parliament that the first arrests had been made in the relation to the incident, and the investigations are continuing.
Scotland Yard, who run the police in London, say the attacker was 52-year-old Khalid Masood.
They say they "believe that Masood acted alone on the day and there is no information or intelligence to suggest there are further attacks planned."
The head of counter terrorism at the Metropolitan Police, Mark Rowley, said: "It is still our belief that this attacker was inspired by international terrorism."
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The BBC's security correspondent Frank Gardner described what happened as the "most serious terrorist attack on London since the 7/7 bombings in 2005".
The following evening, there was a special gathering called a vigil, near to where the attack took place. People came together and lit candles to remember those who lost their lives.
Newsround's guide explains more about the events of that afternoon, who was involved and what happened afterwards.
At around 2:40pm on Wednesday, a single attacker drove a car along Westminster Bridge and hit many pedestrians.
He then crashed the car into railings near the Houses of Parliament, before running towards Parliament where he attacked a police officer.
The attacker was then shot dead by armed police.
Who was involved?
Six people were killed in the incident, including the police officer and a woman who was hit by the car.
The police officer was named as PC Keith Palmer, and the four other victims have been named as Kurt Cochran, Aysha Frade, Leslie Rhodes and Andreea Cristea.
Fifty people were also injured.
The extremist group who call themselves Islamic State - or IS - said that the man who carried out the attack was working for them. But this has not been confirmed by the authorities.
Many people who were in the area when the attack took place were told to stay in their buildings to keep them safe. They were later allowed to go home.
Hundreds of people were evacuated from Parliament and kept safe in Westminster Abbey.
What happens now?
The police are working hard to find out more about what happened.
They have also said there will be more security on the streets, both in London and across the country, to make sure everyone stays safe.
The UK prime minister Theresa May said that the day after the attack, "Parliament will meet as normal, we will come together as normal, and Londoners and others from around the world who have come here to visit this great city will get up and go about their days as normal".
"We will all move forward together," she said.
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