'Multiple' casualties in London 'terrorist' attack
As they rampaged through a busy night-life hub around London Bridge, the trio wore what looked like suicide vests which turned out to be "hoaxes", said Mark Rowley, head of counter-terrorism policing.
"We believe that six people have died in addition to the three attackers shot dead by police," Rowley said.
More than 30 casualties were rushed to hospitals in the area, according to the London Ambulance Service.
Witnesses described a van speeding into pedestrians on London Bridge and then a knife-wielding man sprinting towards a bar packed with revellers enjoying a Saturday night out.
Police responded rapidly with dozens of emergency vehicles and authorities led shellshocked members of the public away from the scene with their hands on their heads.
Police said the first reports of a "vehicle in collision with pedestrians on London Bridge" came at 10:08pm local time (2108) and were quickly followed by "reports of stabbings in Borough Market," at the south end of the bridge.
"Armed officers responded and shots have been fired," police said.
British Prime Minister Theresa May confirmed that the "terrible incident in London" was being treated as "a potential act of terrorism." She will hold an emergency ministerial meeting later on Sunday.
Mayor of London Sadiq Khan condemned the attacks as "barbaric."
The attack comes more than a week after a suicide bomber killed 22 people at a concert in the northern city of Manchester and just days ahead of the June 8 election.
'Wounded people'
Witnesses on London Bridge reported seeing a van mounting the pavement and hitting pedestrians and a man with a knife running.
"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.
The London Ambulance Service said "multiple resources" were being sent to the scene. AFP reporters saw two police helicopters over the area and several roads were shut down.
Will Heaven, managing editor of The Spectator magazine, said on Twitter he saw "two casualties -- one on pavement, one edge of road" and reported seeing armed police on the bridge.
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.
Another witness, who gave his name as 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.
"Then I tried to help people, wounded people."
The incident is reminiscent of the Westminster Bridge attack in March, in which five people were killed and more than 50 wounded.