A van ploughed into pedestrians and several people were stabbed in central London on Saturday, leaving at least 20 casualties in what police called a ‘terrorist’ attack days before a general election.

Armed police opened fire during two “terrorist incidents” at London Bridge and nearby Borough Market, a popular nightspot teeming with bars on a warm summer evening.Police said three attackers drove a vehicle into pedestrians on London Bridge and then stabbed people in Borough Market.

"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.

London's ambulance service said it had taken more than 30 people to hospitals across the British capital after an attack which killed six people late on Saturday.

"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," said Peter Rhodes, London Ambulance Service's assistant director of operations.

Militants drove a van at high speed into pedestrians on London Bridge before stabbing revelers on the street and in bars in the nearby Borough Market area on Saturday night in what authorities described as a terrorist attack.

Armed police rushed to the scene where shots were heard and authorities urged Londoners on Twitter to "run, hide, tell" if they were caught in the attack. The BBC cited police as saying there had been more than one fatality.

Italian photographer Gabriele Sciotto, who was watching the Champions League final at the Wheatsheaf pub in Borough Market, said he saw three men being shot just outside the pub. In his picture, a man wearing combat trousers, with a shaved head and what looked like a belt with canisters attached to it could be seen on the ground with two more bodies behind him.

“There was a van that crashed into the fences on London Bridge. And then there was a man with a knife, he was running. He came down the stairs and went to the bar,” Dee, 26, who was visibly in shock and declined to give her last name, told AFP.

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.

Witnesses on London Bridge reported seeing a van mounting the pavement and hitting pedestrians and a man with a knife running.

"It looked like he was aiming for groups of people. I froze because I didn't know what to do," Mark Roberts, 53, a management consultant, told Reuters. He saw at least six people on the ground after the van veered on and off the pavement.

"It was horrendous," he said.

A taxi driver told the BBC that three men got out of the van with long knives and "went randomly along Borough High Street stabbing people."

Witnesses said people ran into a bar to seek shelter.

"People started running and screaming, and the van crashes into the railing behind. We went toward Borough Market and everyone went inside (the bar). Everyone in the bar started pushing people from the exits," one witness who gave his name as Brian, 32, told Reuters.

Another witness, who declined to be named, his white top covered in blood, described a scene of panic in the bar.

"They hit the emergency alarm. There was a line of people going down to the emergency exit. And then people started screaming coming back up," the 31-year-old said.

"Around the corner there was a guy with a stab wound on his neck ... There was a doctor in the pub and she helped him. They put pressure on the stab wound."

BBC radio said witnesses saw people throwing tables and chairs at the perpetrators of the attack to protect themselves.

“There was a van that crashed into the fences on London Bridge. And then there was a man with a knife, he was running. He came down the stairs and went to the bar,” Dee, 26, who was visibly in shock and declined to give her last name, told AFP.

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.

Three men arrested over Manchester attack

“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.

Alex Shellum at the Mudlark pub near the scene of the attack said a woman had come into the bar “bleeding heavily from the neck”. “It appeared that her throat had been cut,” he told the BBC. 

Another witness, who gave his name as Alessandro, also 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.”Then I tried to help people, wounded people.”

Husband and wife Ben and Natalie told BBC Radio 5 Live they were outside Borough Market when they witnessed the incident there.

Ben said: “We saw people running away and then I saw a man in red with a large blade, at a guess 10 inches long, stabbing a man, about three times.

“It looked like the man had been trying to intervene, but there wasn’t much he could do. He was being stabbed quite coldly and he slumped to the ground.”

Ben said the man then walked towards the Southwark Tavern. He said they saw a metal chair being thrown towards the man.

“Then we heard three gunshots, definitely gunshots, and we ran.”

Britain's Sun newspaper said seven people were feared killed and two attackers were shot dead by police near London Bridge; but there was no immediate confirmation of this. Sky news said police were unable to confirm all attackers were accounted for.

The attacks come days ahead of a June 8 election and less than two weeks after a suicide bomber killed 22 people at a pop concert by U.S. singer Ariana Grande in Manchester in northern England. There was no immediate claim of responsibility.

The BBC showed a photograph of two possible London attackers shot by police, one of whom had canisters strapped to his body. Hours after the attack the area remained sealed off.

The London ambulance service said at least 20 people had been taken to six hospitals across the city. Three major London hospitals said that they were on lockdown to keep patients and staff safe.

A Reuters reporter said some time after the attack began that he had heard loud bangs near the Borough Market area.

Streets around London Bridge and Borough Market, fashionable districts packed with bars and restaurants, would have been busy with people on a Saturday night out. BBC showed dozens of people, evidently caught up in the attack, being escorted through a police cordon with their hands on their heads.

ISLAMIC STATE CALL

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.

Similar attacks, in Berlin, Nice, Brussels and Paris, have been carried out by militants over the past couple of years.

"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.

London mayor Sadiq Khan said an attack in central London late on Saturday was a "deliberate and cowardly attack", and that he would take part in a security meeting with Prime Minister Theresa May later on Sunday.

"We don't yet know the full details, but this was a deliberate and cowardly attack on innocent Londoners and visitors," Khan said. "I condemn it in the strongest possible terms. There is no justification whatsoever for such barbaric acts," he added. 

London's Thames river police said it was working with the lifeboat rescue service to help evacuate people caught up in the attack, described by police as a terrorist incident.

U.S. President Donald Trump took to Twitter to offer U.S. help to Britain. The White House said he had been briefed on the incidents by his national security team.

One woman told Reuters she saw what appeared to be three people with knife wounds and possibly their throats cut at London Bridge at the Thames river. Reuters was unable to immediately verify her account.

STABBINGS ON THE STREET

The incident bore similarities to a March attack on Westminster Bridge, west of London Bridge, in which a man killed five people after driving into a crowd of pedestrians before stabbing a police officer in the grounds of parliament.

Several witnesses also reported hearing gunshots around London Bridge.

"We were in an Uber (taxi) going toward London Bridge and suddenly we saw people running. The Uber stopped, we asked people what was going on – people said there was shooting," said Yoann Belmere, 40, a French banker living in London.

"Now the area is completely closed with police cars going one way and ambulances going the other," he told Reuters.

A witness told CNN two men had entered a restaurant in the Borough Market area and stabbed two people inside. He said a waitress was stabbed in the throat and a man was stabbed in the back.

The Manchester bombing on May 22 was the deadliest attack in Britain since July 2005, when four British Muslim suicide bombers killed 52 people in coordinated attacks on London's transport network.