London (AFP) - Police said a "major incident" was underway in central London as they responded to reports Saturday of a vehicle ploughing into pedestrians on London Bridge, and stabbings and shots being fired nearby.

"From 2208 hours (2108 GMT) officers responded to reports of a vehicle in collision with pedestrians on London Bridge," police said in a statement.

"Officers have then responded to reports of stabbings in Borough Market," at the south end of the bridge. "Armed officers responded and shots have been fired".

"Officers are now responding to an incident in the Vauxhall area," which is also on the south side of the River Thames and home to the MI6 foreign intelligence agency.

Witnesses on London Bridge reported seeing a van mounting the pavement and hitting pedestrians.

The London Ambulance Service said "multiple resources" were being sent to the scene.

The police were urging the public to run to a place of safety and hide if they cannot.

"Please remain calm, but be alert and vigilant. We are using all necessary skills and resources," police said.

Will Heaven, managing editor of The Spectator magazine, said on Twitter that he saw "two casualties -- one on pavement, one edge of road" and reported seeing armed police on the bridge.

- 'Wounded people' -

BBC reporter Holly Jones, who was there at the time of the incident, said she saw a van driven by a man travelling "at about 50 miles (80 kilometres) an hour".

She said about five people were being treated for injuries after the vehicle mounted the pavement and hit them.

"There's several police boats with torchlights searching the Thames at the moment," she told BBC radio.

She added that she saw a man, who had his shirt off and was in handcuffs, being arrested by police.

Another witness, Alessandro, told BBC radio that he saw a van strike several people on London Bridge.

"I saw this van going left and right, left and right, trying to catch as many people as he could. And people just tried to get out of the way of the van.

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"Then I tried to help people, wounded people."

Of the casualties, he said: "Three of them, yes (were conscious), and one guy was not talking at all, was just, like, down."

"There were five or six people that we tried to help, they were young people."

His friend Giovanni said he contacted police and the ambulance services, which arrived within around two minutes.

The incident is reminiscent of the March 22 Westminster Bridge terror attack in the city in which five people were killed and more than 50 wounded.

The assailant in that attack, 52-year-old British Muslim convert Khalid Mahmood, rammed his car into pedestrians before crashing into the barriers surrounding parliament and then stabbing a police officer to death.

He was then shot dead by police at the scene. Investigators later described the lone-wolf attack as "Islamist related terrorism".

It also comes less than two weeks after 22 people, including seven children, were killed in a suicide bombing at the end of a pop concert at the Manchester Arena.

Manchester-born Salman Abedi, a 22-year-old of Libyan origin, was named as the Manchester bomber.