The attack began when the driver of a car struck bystanders and three police officers on the Westminster Bridge, London's Metropolitan Police said. The car then crashed near the Houses of Parliament and at least one man armed with a knife attacked an armed officer who was guarding Parliament, police said.
The suspected attacker was shot by police.
French students were among the injured, according to French officials. A seriously injured woman was pulled from the River Thames and is receiving medical treatment, an official with the Port of London Authority told ABC News. Tobias Ellwood, a member of Parliament, was seen giving first aid to one of the victims.
Authorities said they received several different reports today, including a person in the River Thames, a car involved in a collision with pedestrians and a man armed with a knife.
An eyewitness told the BBC that someone driving a car on Westminster Bridge appeared to hit bystanders.
Witness Richard Tice told ABC News that he saw injured people on the pavement. According to Tice, the car appeared to have jumped the curb, knocking people over.
Additional officers were deployed across London today and the public was asked to remain vigilant.
After the attack Parliament was placed on lockdown.
As police swarmed the area, Tom Peck, a British journalist, said that he heard a loud bang, lots of screaming and then several gunshots from his office in London.
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."
This story is developing. Please check back for more updates.