Seven people were killed in a terror attack in central London late Saturday before police shot three suspects dead, the Metropolitan police said. All three were shot dead.
May said the attacks weren't directly connected, but that "terrorism breeds terrorism" and attackers copy one another.
It was the third deadly attack on British soil since March, when a man drove his vehicle into pedestrians on Westminster Bridge in central London and stabbed a policeman to death outside parliament.
Witnesses described how passers-by threw chairs and beer glasses at the attackers in an attempt to stop them.
Reports said the men then jumped out of the van and began stabbing people nearby.
The suspects are then believed to have jumped out the van and proceeded on foot to nearby Borough Market, a popular nightlife spot, where witnesses said they produced knives and slashed indiscriminately at customers in restaurants and bars.More news: London terror: Member of public caught in police fire
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"He always sets up community groups in terms of the homelessness that is happening here, so to hear that he is now complicit with this, it is very hard to stomach", Pettit said.
French President Emmanuel Macron said French citizens were among the victims. After the Westminster terror attack in March the US President's son, who is named Donald Trump Jr, was called "disgraceful" after goading Mr Khan on social media and quoting him out of context. Grande and other stars are scheduled to perform a benefit concert for victims under tight security in Manchester Sunday night. "Things need to change", May said.
"We cannot and must not pretend that things can continue as they are", May said, calling for a strengthened counter-terrorism strategy that could include longer jail sentences for some offenses and new cyberspace regulations.
She repeated calls for global action to combat extremist content online, a message she took to the G7 leaders summit last week.
"While we have made significant progress in recent years, there is, to be frank, far too much tolerance of extremism in our country", May said during her address, declaring "enough is enough".
She said most political parties had suspended campaigning until Monday as a mark of respect and that the general election would still go ahead on Thursday, June 8. Trump tweeted Saturday after the attack. "If we don't get smart it will only get worse", Trump said. "GOD BLESS!" - and highlighting his thwarted ban on travellers from six mainly Muslim countries.
'There were three men wearing what appeared to be suicide belts.
Saturday's attack was reminiscent of another on Westminster Bridge on March 22, when Khalid Masood, 52, drove a auto into pedestrians, killing four people.