The attack comes just weeks after suicide bombing killed 23 in Manchester, and it's the third terrorist attack in England in under three months.
But, with the incident still unfolding and the number of casualties unclear, these are questions for another day.
A van rammed several pedestrians on London Bridge on Saturday evening, according to an eyewitness.
She said about five people were being treated for injuries after the vehicle mounted the pavement and hit them.
Gabriele Sciotto, a photographer, told the BBC that he had run toward the scene of the attacks and that "in 2-5 seconds, [police] shot all three men down". He described the incidents as "brutal" terror attacks.
One witness described seeing five assailants and several people said they were locked inside pubs and restaurants by police and had to come out with their hands on their heads. "It was a rampage really".
Bartender Alex Martinez said he hid in a garbage bin for a half hour when a man stormed the restaurant where he worked, which was almost full. Using a vehicle as a weapon is a method used in Nice France; in the Berlin, Germany Christmas market attack; in the Westminster bridge attack at UK Parliament, and now on London Bridge. He said he threw chairs, glasses and bottles at the attackers in a bid to stop them.
"Around the corner there was a guy with a stab wound on his neck". There are reports that law enforcement is looking for three suspects who may be armed.
Armed police were called to the scene and within eight minutes, the three attackers were shot dead.More news: Report auto hits pedestrians on London Bridge, some said hurt
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Police declared both incidents terrorist attacks.
Prime Minister Modi took to Twitter to express his anguish.
London Mayor Sadiq Khan called it "a deliberate and cowardly attack on innocent Londoners". I condemn it in the strongest possible terms. "There is no justification whatsoever for such barbaric acts", he added.
Prime Minister Theresa May planned to chair an emergency security Cabinet session Sunday to deal with the crisis.
Leaders from across the globe have shown an outpouring of support and condolences for the United Kingdom after a terrorist attack struck the nation's capital Saturday night, killing at least six people and wounding some 48 others. Fellow French citizens are among the injured. They all have paramedics assisting them at the moment'. From the window, I could see an injured person being treated by emergency personnel.
Ms Jones said: "She told me in French: 'I don't know where those two other people are.' So the police are checking the Thames". Pedestrians near the bridge said they were ordered by police to run, and video footage showed people fleeing in a panic.
Fourteen people injured in the attack were taken to King's College Hospital, in Denmark Hill, south London.
They then emerged and attacked some of the people they had hit with the van before heading to Borough Market, where the pubs and restaurants were packed with Saturday night crowds, including many watching the Champions League final between Juventus and Real Madrid in Cardiff.
"One patient has been discharged".