A man went flying some 20 feet in the air as a white van crashed into a group of people on the London Bridge, a witness said on Sunday.

"It knocked a person about 20 feet in the air and within my line of sight there must have been about five or six people on the ground. They looked either dead or dying," Efe news quoted Mark as saying.

Six civilians died in the Saturday night terror, which also included an attack in the nearby Borough market. Police shot dead three suspects. 

Other witnesses reported seeing a man with "a big knife" in the Borough market and a vehicle ramming pedestrians on the London bridge.

One of them, Neal Tate, said he saw two or three young men in their 20s being detained on Borough High Street when he was on his way home.

Alex Shellum, another man in a London pub near the bridge, saw a girl in her 20s stagger into the pub, bleeding a lot.

"It appeared her throat had been cut," recalled Shellum, adding that the pub was then ordered closed to the public. Armed police asked them to leave the area.

According to Scotland Yard, the incident on the London bridge occurred at 9 p.m. when many people were enjoying a pleasant evening in good weather. After hitting the pedestrians, the vehicle headed towards the Borough market full of bars and restaurants.

After the attack, the suspects abandoned the vehicle and proceeded to stab people in the area, including a police officer who was seriously injured.

Armed police responded "very quickly and bravely and confronted three male suspects, who were shot and killed on Borough market", Efe quoted an official statement as saying.

The BBC showed an image of two men lying on the ground, allegedly after being shot by police, one of them with cans attached to the body.

Another violent incident with an edged weapon took place simultaneously in Vauxhall district south of London but police dissociated it from terror.

The attacks come two weeks after the Manchester terror attack at the concert of American pop singer Ariana Grande that killed 22 people.