LONDON (CBS/AP) – British police say four people were killed in today’s London attack outside Britain’s Parliament, including one police officer and the attacker. Twenty other people have been injured.

British port officials say they pulled a woman from the Thames River following the incident on Westminster Bridge.

Police are treating the attack as a “terrorist incident.” It prompted a lockdown of the Houses of Commons and other nearby buildings.

London police sources tell CBSN around 3 p.m. London time, an attacker driving a truck hit several people standing on Westminster Bridge and then crashed the vehicle into a fence surrounding Parliament buildings. He then allegedly got out of the vehicle and ran towards an unarmed officer, stabbing him repeatedly.

Police standing nearby saw the stabbing and shouted for the man to stop, but he reportedly rushed towards them. Officers then shot the man repeatedly before he fell to the ground.

“I saw a thick set man wearing black clothes run through the gate where cars go in and started striking or stabbing a police officer. He then ran towards and entrance…he got about 15 feet in where police yelled to him to stop,” Daily Mail reporter said Quentin Letts told CBSN. “The attacker was shot 2-3 times. These two armed officers adopted a shooting stance and shouted words at the man running toward him and he was shot.”

“There were people across the bridge. There were some with minor injuries, some catastrophic. Some had injuries they could walk away from or who have life-changing injuries,” said  Colleen Anderson of St Thomas’ Hospital.

A senior police commander says the attack was declared a terrorist incident and “a full counterterrorism investigation is underway,” said Commander B.J. Harrington.

He says additional police officers, armed and unarmed, will be deployed across London during the evening rush hour as part of efforts to keep people safe.

People began leaving the Parliament about two hours after the incident.

Poland’s former foreign minister, Radek Sikorski, says he was in a taxi leaving Westminster and was checking his email when he heard something like a car crash, “something like a car hitting metal sheet.”

“I look up and I see that a person is lying on the pavement. I started my camera and I saw more people lying on the street and on the pavement,” Sikorski said on Poland’s TVN24.

“People started running up to them. I saw one person who gave no sign of life, another man was bleeding from his head. In all, I saw five people who were at least seriously injured,” he said.

“The taxi driver immediately called the emergency number. I heard ambulances within two or three minutes, so the rescue action was really very quick. There is a hospital near there.”

“It all happened so fast that there was no time to get scared,” said Sikorski who posted his video on Twitter.

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