London police confirm attacks in London Bridge and Borough Market are terrorist ...

Police cars close off north entrance to London Bridge next to monument station after van ploughed into pedestrians. (Wang Bo / China Daily)

British security services were scouring the streets of central London for attack suspects after a series of incidents which have left at least one person dead and an unspecified number of people seriously injured.

London's Metropolitan Police said the attacks in London Bridge and Borough Market are terrorist incidents. They are dealing with two incidents amid witness reports of a van mounting a pavement and running down pedestrians on London Bridge. Witness later said that police fired rounds next to Borough Market.

A spokesman for the Prime Minister's office at 10 Downing Street said Theresa May was being kept up to date on the incident and was heading for her office.

Earlier UK Prime Minister Theresa May says the "terrible incident" in London is being treated as "potential act of terrorism".

Police then said they were responding to a third incident at Vauxhall, around three miles away from London Bridge. It was later declared by the police that the incident at is a stabbing and is not connect to the incidents at LondonBridge and Borough Market.

One witness said that three men got out of the van on London Bridge with knives and attacked passers-by.

Armed police entered bars and ordered customers to lie on the ground. Police are continuing to evacuate the area.

BBC reporter Holly Jones, who was on the bridge at the time of the incident, said she saw the van, driven by a man, veered off the road and hit pedestrians while travelling at 50 miles an hour.

Police have issued a warning to Londoners to "run, hide and tell" if they are in danger. This means people should run and hide at a sign of danger and then inform police.

Borough Market is a popular destination, with a number of restaurants in the area.

The incident follows an attack on Westminster Bridge when Khalid Masood killed four pedestrians on Westminster Bridge before stabbing a police officer to death, Masood was then shot dead by police.

The incident also coincided with a concert in Manchester to commemorate the victims of last week's bombing attack in Manchester which killed 22 people.