Two people have allegedly been arrested after reports of a car ploughing into pedestrians on Westminster Bridge.
Bernard Cazeneuve, the Prime Minister of France, has confirmed a number of French students are among the casualties.
A police officer has also been stabbed and a suspected assailant shot outside Parliament moments after the incident.
Several victims are being treated for "catastrophic" injuries after the attack, according to a junior doctor speaking to the Press Association.
Colleen Anderson, a junior doctor at St Thomas' Hospital, told reporters a female pedestrian had died.
Mitchell Spree, a van driver, told The Independent he was driving along Embankment when he saw people being evacuated from nearby buildings.
He said: "Then we pulled on to the bridge. A lady was laying at the entrance to the bridge. There was about five more people.
"She was crying. She was speaking to the paramedic. I don't know what the others were like. The police asked us to leave our van. It's still on the bridge.
"It's shocking."
Ismael Hassan, an eyewitness just metres from the incident, said he saw a 4x4 swerving up the bridge, hitting about 10 people.
Various witnesses have confirmed that a body was seen floating in the water near Westminster Bridge after the attack.
At least three gun shots were heard by those inside Westminster, and all proceedings in the House of Commons have been suspended.
Police are treating it as a "terrorist incident" until they find out otherwise.
Matt Haikin, 44, said he was in shock after seeing the aftermath of the crash on the bridge.
He said: "I just saw a car that had clearly driven off the road into the fence outside Parliament.
"As I went past I noticed there was a body next to it and quite a lot of blood and people standing around.
"Fairly shortly after I heard some shots, at which point it was clear it wasn't just an accident, something else was going on."
He then moved to look through the Palace of Westminster gates and saw "a lot of people, people in uniform, I think I saw a couple of bodies on the ground, I couldn't tell you if they'd been asked to lie down or if they were injured".
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