Eyewitnesses told LBC of the terrifying scenes they saw during the incident on London Bridge.
Also Saturday, a man with a "massive knife" entered a restaurant at Borough Market, just south of the bridge, and stabbed two people inside, a witness told CNN on Saturday.
A witness told the BBC she saw a speeding white van veering into pedestrians.
Describing what appeared to be canisters strapped to the man's chest, he said: "The first sight at thought it wasn't anything real, it didn't look real to me but I'm not like the person who could honestly say if they were real or not because I have no experience".
He told Sky News: "It was about 10.15pm I was in the back of an Uber cab driving over London Bridge.there was somebody down on the pavement with a small crowd around them clearly very concerned".
London Bridge Tube station was closed and London Bridge was closed in both directions.
The force tweeted a warning telling people in the area to run to safety, hide and then call police if it is safe to do so.
LANGFITT: Absolutely, and I think that coming - three incidents like this, you know, there was a long stretch where London did not face big sort of terrorist attacks.
"A van came from London Bridge itself, went between the traffic light system and rammed it towards the steps".
"After that they were nearly coming towards me and I tried to run away".
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The assailant, Khalid Masood, 52, of West Midlands, reportedly had a criminal record and may have had connections to violent extremism, British Prime Minister Theresa May said.
Other witnesses reported seeing as many as three attackers with knives. The patrons sheltered in the basement and police have arrived at the scene, the witness said.
Medics treated people lying injured on the ground near the market as shocked people cried and shouted around them.
Simon Thompson told Sky News that he was just outside Borough Market when he saw crowds fleeing. "Police told us to get out and then there was more gunfire".
At about 1.30am up to four blasts were heard near to London Bridge Street. It said the blasts were believed to be controlled explosions carried out by police.
Armed officers flooded the busy area of the city after a van struck multiple people on the bridge.
The Royal Oak pub, near the attack site, opened its doors to people evacuated from hotels inside the area cordoned off by police. Witnesses also reported seeing people being stabbed by at least one man.
If confirmed as terrorism, this will be the third attack to hit Britain in as many months.
On May 22, a suicide bomber killed 22 people and injured dozens at an Ariana Grande concert in Manchester.
Grande visited victims of the bombing in the hospital Saturday and is scheduled to perform at a star-studded benefit concert in Manchester on Sunday.
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