At least seven people were killed and 30 others injured on Saturday in two terror attacks at central London landmarks in which police shot dead three suspects, the third terror incident to strike the UK this year. So far, 12 people have been arrested on suspicion.

The attack that took place at London Bridge and Borough Market comes just days ahead of the general election on 8 June.

The first incident was reported at London Bridge when a van swerved off the road into a crowd of pedestrians, BBC reported.

A white van driver came speeding probably about 50 mph veered off the road into the crowds of people who were walking along the pavement, an eye-witness was quoted by the report.

Within minutes, there were reports of a second attack at Borough market on the South Bank of the Thames where three men wielding knives 10 inches long began attacking passersby, even entering one restaurant to attack night diners.

One report suggested throats had been slashed by the marauders, the report said.

Six people were killed in the attack, and at least 30 were taken to hospital, the Metropolitan police said.

"We have now taken over 30 patients to five hospitals across London following the incident at London Bridge. A number of other patients were treated for less serious injuries at the scene," Peter Rhodes, London Ambulance Service Assistant Director of Operations, said in a statement.

"We declared a major incident and continue to work closely with other members of the emergency services at the scene," it said.

The three assailants shot dead by police were wearing fake suicide vests, police said today.

"The suspects were wearing what looked like explosive vests but these were later established to be hoaxes," Britain's head of counter-terrorism Mark Rowley said.

Prime Minister Theresa May will chair a Cobra (Cabinet Office Briefing Rooms) meeting.

May said that the "terrible incident" in London is being treated as a "potential act of terrorism".

The prime minister said: "Following updates from police and security officials, I can confirm that the terrible incident in London is being treated as a potential act of terrorism".

It was the third terror attack to strike the UK this year, after a man drove a car into pedestrians on Westminster Bridge in March and the bomb attack on an Ariana Grande concert in Manchester two weeks ago.

On 22 May, 22 people were killed and 116 injured in a suicide bombing at Manchester Arena.

The attacker, identified as 22-year-old Salman Abedi, detonated a home-made bomb in the arena's foyer as crowds were leaving a performance by US singer Ariana Grande late in the evening.

On 22 March, six people died, including the attacker, and at least 50 people were injured in an attack near the Houses of Parliament.

Attacker Khalid Masood first drove his car at pedestrians on the pavement on Westminster Bridge before crashing into a perimeter fence. Masood, armed with a knife, then left the vehicle and ran towards Parliament, where he was shot dead by police.