Six people were killed and more than 40 injured when three knife-wielding men attacked a restaurant after mowing down crowds with a van in what London police say was a terrorist attack in the British capital.
London's mayor, Sadiq Khan, said in a statement: "My thoughts are with everyone affected, and I'd like to thank the fearless men and women of our emergency services who were first on the scene and will be working throughout the night".
A man who gave only his first name, Ben, told BBC radio that he "saw a man in red with a large blade, at a guess 10 inches long, stabbing a man, about three times" near Borough Market.
Major parties suspended national campaigning Sunday out of respect for the victims, although the right-wing UK Independence Party said it would keep campaigning to show the extremists they couldn't sidetrack democracy.
They joined the Conservative Party in halting campaigning ahead of the June 9 vote.
The Manchester bombing 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.
Prime Minister Theresa May called an emergency security cabinet session Sunday to deal with the crisis.
A vehicle veered off the road and mowed down pedestrians on London Bridge, turning a summery Saturday night into a scene of panic and chaos.
In a statement denouncing the "abominable and cowardly" attack, Macron said France will continue fighting "terrorism with all our strength alongside Britain and all other countries concerned". Police officers yelled at people to run from the area.
"I can confirm that the awful incident in London is being treated as a potential act of terrorism", Prime Minister Theresa May said in a statement as events unfolded.
Witness Mark Roberts, who was on the bridge at the time of the attack, told CNN the van was traveling south across the River Thames at a speed and was swerving as it struck several people, knocking one person "about 20 feet into the air".
Duterte's spokesman Ernesto Abella said "acts such as these touched not only the people of London, they also touched all peace-loving people in the global community". "We stand together against terror".
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Gerard Vowls was in a pub near London Bridge and said he threw chairs and glasses at the attackers.
The US State Department said America was ready to provide assistance to United Kingdom authorities.
At least two French citizens have been injured in the attack, the French president's office said.
"London is one of the greatest cities in the world, a place where people from every continent have made their home".
Speaking outside Downing Street, she said: "Violence can never be allowed to disrupt the democratic process".
"We must also thank those in our emergency services who yet again put themselves in danger to protect others".
The attackers "had no clue what they were doing".
"We believe three people were involved, but we still have got some more inquiries to do to be 100 percent confident in that", he said.
And Mr Corbyn said: "We are all shocked and horrified by the brutal attacks in London".
The ministry stressed that London's terrorist attacks reaffirm the urgent need for concerted worldwide efforts to combat terrorism, calling for joint cooperation and addressing this serious scourge that threatens the security and stability of countries all over the world.
He said in a brief statement after returning from Europe that his thoughts are with the families of the deceased and offered prayers for the injured.
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. Police said they were treating the incidences as "terrorist" attacks.