At least six people were killed by militants who drove a van at high speed into pedestrians on London Bridge and then stabbed revellers in the nearby Borough Market area on Saturday.

For the main story on the attack:

All three attackers were shot dead by police. More than 30 injured people were taken to hospital.

The following is what is known so far:

- Three attackers drove a white van at high speed into pedestrians walking on London Bridge, police said.

A witness told Reuters that at least six people were hit on the bridge.

"It looked like he was aiming for groups of people. I froze because I didn't know what to do," Mark Roberts, a 53-year-old management consultant, told Reuters. "It was horrendous."

Police said they responded to the incident on London bridge at 2208 (2108 GMT).

- The men armed with knives left the vehicle and attacked people in the nearby Borough Market area on the south side of the Thames, witnesses said.

Witnesses said the attackers stabbed people in the street and in pubs and restaurants in Borough Market.

- Police said armed police had responded to the reports of stabbings in Borough Market and shots had been fired.

"Armed response officers then responded very quickly and bravely and confronted the three male suspects who were shot and killed on Borough Market," Mark Rowley, Britain's top anti-terrorism officer, said.

"The suspects had been confronted and shot by police within eight minutes of the first call. The suspects were wearing what looked like explosive vests but these were later established to be hoaxes," he said.

The men were wearing what appeared to be canisters attached to their bodies. One of them was shown in a photograph lying on the ground outside the Wheatsheaf pub.

Another man, was also reported to be an attacker, was pictured on the ground a short distance away. Witnesses said they heard gunfire in the area.

The police said the vests were later established to be hoaxes. A Reuters reporter nearby said he heard loud bangs afterwards that may have been controlled explosions.

- Terror attack? At 0025 British time on Sunday the incidents at London Bridge and Borough Market were declared as terrorist incidents.

"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," Prime Minister Theresa May said just over four days before voting is due to begin in a June 8 national election.

"This is a fast moving investigation," May said. "I want to express my huge gratitude to the police and emergency services who are on the scene. Our thoughts are with those who are caught up in these dreadful events."

May will chair a meeting of Britain's "COBR" emergency response committee on Sunday morning. Intelligence and security chiefs as well as senior ministers and officials attend the meeting.

London's Metropolitan Police Counter Terrorism Command is leading the investigation.

- Who? It was not clear who might be responsible for the attack.

Islamic State earlier on Saturday sent out a call on instant messaging service Telegram urging its followers to launch attacks with trucks, knives and guns against "Crusaders" during the Muslim holy month of Ramadan.

- The attack came less than two weeks after a suicide bomber Salman Abedi blew himself up at a concert in Manchester on May 22, killing at least 22.

Britain raised its terror threat to the highest level of "critical" and deployed troops on the streets on May 23, a day after the Manchester suicide attack. The critical level means another attack could be imminent. It was reduced to "severe", which means an attack is highly likely, on May 27.

On March 22, Khalid Masood ploughed a rented car into pedestrians and stabbed a policeman to death before being shot dead. Five were killed by the attacker.

- The United Kingdom is due to hold a national election on June 8.

- U.S. President Donald Trump tweeted: "Whatever the United States can do to help out in London and the U.K., we will be there - WE ARE WITH YOU. GOD BLESS!"

- A separate incident in Vauxhall, two miles away, was not related to the other two incidents police said.