Seven people were killed, while 48 were injured.
Prime Minister Theresa May's office said she was "in contact with officials and is being regularly updated on the incident at London Bridge". "But the whole of our country needs to come together and confront this extremism", she said.
Although there was no immediate claim of responsibility for the attack, May said it was tied to Islamic extremism.
People from around the world were caught up as hundreds cowered in pubs and restaurants, barricading themselves inside as the attackers stalked the streets.
Saturday's attack, in which three men drove a van into pedestrians on London Bridge before stabbing people in nearby bars and restaurants, occurred five days before a parliamentary election.
Thirty-six people remain hospitalized, according to The Associated Press, with 21 in critical condition.
Anyone with images or footage of Saturday night's attack have been urged to share it with counter-terror police to assist their investigation. As I think you are aware there are searches ongoing in east London, and 12 arrests have been made. More homes in the area were being searched. But then he went and hit one of the attackers on the head with a crate.
The force said armed officers were sent to the scenes and shots were fired.
A bystander was also wounded by the gunfire, but the civilian's injuries were not believed to be critical.
Rowley lauded the courage of officers who ran toward the attack as it unfolded.
"The bravery he showed was outstanding and makes me extremely proud", Crowther said.
One Spanish citizen was injured, the embassy said, but is not in a life-threatening condition.
He said: "My message to Londoners and visitors to our great city is to be calm and vigilant today". "GOD BLESS!" But he also took shots at political correctness and gun control. "If we don't get smart it will only get worse", Trump tweeted.More news: Poll Projection Suggests British PM May Could Lose Upcoming Election
More news: Pruitt: Trump has 'indicated that climate changes'
More news: Muguruza in third round, Goffin retires at French Open
He said he was "heartbroken to learn that a Canadian is among those who lost their lives".
The U.S. Department of Homeland Security issued a statement late Saturday saying "At this time, we have no information to indicate a specific, credible terror threat in the United States". K., we will be there - WE ARE WITH YOU.
The van mounted the sidewalk on the bridge and struck pedestrians.
"It was horrific", he said.
Ms Owen said there were 30 to 40 people hiding in the upstairs of the pub with her, that they were "terrified" and with the sound of gunshots "everyone jumped to the floor".
The death toll of seven did not include the three attackers. Some of the wounded pleaded for help, Ansah said.
"I believe in many ways the Muslim-American community is better integrated into our society".
Britain has weathered two other terror attacks in recent months. Police later said Masood had no links to extremist groups such as ISIS or al-Qaeda.
On May 22, a British suicide bomber killed 22 people and injured dozens at an Ariana Grande concert in Manchester. Abedi died at the scene.
He also criticized Khan for his response to the attacks.
In a statement, Simon Milner, director of policy at Facebook, said the social network giant wants to "provide a service where people feel safe".
"Having visited the officer in hospital shortly after he was admitted for treatment, I was able to hear his account of what happened last night", Crowther said, "Although he is seriously unwell, he was able to recount how he faced the attackers armed only with his baton, outside London Bridge station".
The ruling Conservatives and the main opposition Labour party suspended national campaign events for the day, although local campaigning will continue. It is not clear if campaigning will resume in the final days before the vote.