The 12 arrests were made in the ethnically diverse east London suburb of Barking.
Home Secretary Amber Rudd said Sunday police believe the three men shot dead by police were the lone perpetrators of the attack and therefore raising the threat level to the top at "critical" was not warranted. "They will never succeed", Clark said.
Eyewitnesses described harrowing scenes as the attackers' white van veered on and off the bridge sidewalk, hitting people along the way, and the three men then ran into an area packed with bars and restaurants, stabbing people indiscriminately.
"Eight police firearms officers discharged their weapons". Other officers with guns arrived a short time later and fatally shot the assailants, who wore what appeared to be bomb-laden vests that turned out to be fake.
As it did after the events in Westminster and Manchester, the Muslim Council of Britain (MCB) issued a statement that strongly condemned Saturday's attack.More news: Dillon takes first Cup victory in delayed Charlotte race
Police have foiled 18 terror plots since 2013 and arrest around one person every day in counter-terror raids, Ms Dick said.
One eyewitness said the attackers screamed "this is for Allah" as they stabbed people.
Conservative politician Penny Mordaunt tweeted the transcript of Khan's interview and said: "I'm standing with resilient London and him". "I was just trying to get their attention by throwing things at them".
Facebook statements posted by her fiancé's brother and sister suggested that Archibald had been struck and killed by the van while walking on London Bridge with her fiancé.
Another witness called Eric told the BBC he had seen three men get out and thought they were going to help.More news: United Kingdom vote looms, Tories confuse tax issue
The president notes the attackers used a van to slam into pedestrians and then assaulted people with knives.
"It was really scary", he said.
A moment of silence was also held in Manchester, where American pop singer Ariana Grande returned to headline a concert to raise money for the victims of the May 22 suicide bombing.
Prime Minister May, who served as interior minister for six years before taking office after the Brexit vote last summer, said Britain's response to the terror threat must change.
With the United Kingdom national elections due on June 8, the country's major political parties temporarily suspended campaigning but May says the vote will take place as scheduled because "violence can never be allowed to disrupt the democratic process".More news: Jared Kushner, Ivanka Trump to Review DC Life Every 6 Months
Key questions remain, including whether the attackers had support or were part of a larger network, and whether the attacks were coordinated.
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