Police shot and killed the three attackers, and May said their identities would be released "when progress in the investigation permits".
With the London attack dominating attention, a reduction in the number of police officers in England and Wales by nearly 20,000 during May's six years as interior minister from 2010 to 2016 shot to the top of the election agenda.
"If you're going to see something and say something, it has to be followed by, do something", she said.
British Prime Minister Theresa May on Sunday blamed "evil" Islamist ideology and vowed to crackdown on extremist content online worldwide, warning that attackers were "copying one another".
Following the third militant attack in Britain in less than three months, May said Thursday's election would go ahead and that Britain had been far too tolerant of extremism. On Monday, the president tweeted: "People, the lawyers and the courts can call it whatever they want, but I am calling it what we need and what it is, a TRAVEL BAN!"
The Islamic State group claimed responsibility.
One Canadian national and one Frenchman were among the fatalities and seven French citizens were among the injured.
Markets fluctuated on Monday morning but hovered close to records set on Friday after a terror attack in London over the weekend.
In an appearance on NBC's "Today Show", Conway said people should pay attention to what the president is doing, saying people in England had tried to inform authorities about the terrorists before the attacks happened.
"They kept coming to try to stab me. they were stabbing everyone".
"The majority of the threat we are facing does not appear to be directed from overseas", Dick told the BBC broadcaster, although stressing that there was "an global dimension" in three recent attacks in the United Kingdom.
"I have heard some of this stuff and it's very radical", he added.
He was stabbed in the face, head and leg and taken to hospital but his injuries are not thought to be life-threatening.
A Frenchman was killed in the attack, Foreign Minister Jean-Yves Le Drian said.
A vigil for the victims will take place at nearby Tower Bridge on Monday evening.
Masood, armed with a knife, then left the vehicle and ran towards Parliament, where he was shot dead by police.
Grande, who headlined a benefit concert in Manchester on Sunday, alongside stars including Pharrell Williams and Justin Bieber, tweeted that she was "Praying for London".
After the attack on Manchester Arena, Tory government ministers were reported to be planning to enforce widespread new powers that would force USA tech firms like Apple, Facebook and Whatsapp to remove encryption from messaging services like Facebook and WhatsApp. She chaired a meeting of the government's emergency response committee, COBRA, on Monday to discuss the response to the attack.
The 21-year-old was enjoying a break from her job as a nanny with friends when three terrorists ploughed a van into pedestrians as the pair walked across the bridge. She said, "I believe it is right that the United Kingdom is engaged in taking on and defeating groups like ISIS and their like around the world".
The Conservatives, Labour and the Scottish National Party all suspended campaigning Sunday for the second time this election. "GOD BLESS!" - and highlighting his thwarted ban on travellers from six mainly Muslim countries.