She said she wanted to make it easier to deport foreign terrorist suspects and restrict their movements if there was not enough evidence to prosecute them.
May also called for longer prison sentences for people convicted of terrorist offences and promised to make it easier to deport foreign terrorists back to their home countries.
"We will not defeat terrorism by ripping up our basic rights and democracy. but by our communities, our vigilance and by police action to isolate and detain those who would wish us harm", he said, responding to May's comments.
Both the Conservative and Labour parties, the U.K.'s largest parties, suspended campaigning Monday in the wake of London terror attack.
At a news conference shortly after the London attack, May said "enough is enough" and vowed to crack down on extremism.
"I can say that with this authority".
"I mean doing more to restrict the freedom and movements of terrorist suspects when we have enough evidence to know they are a threat, but not enough evidence to prosecute them in full in court", May said.
"If our human rights laws stop us from doing it, we will change the laws so we can do it", she said to cheers and applause at an election rally", she added.
He also said ministers needed to consider further restrictions on terrorist suspects.
"We have been very careful with regard to lawfulness and ensuring that, wherever possible, Muslims do not feel discriminated against", said Buckingham University professor of security and intelligence Anthony Glees. "I will listen to what they think is necessary for us to do".
Labour has immediately cried foul, claiming another manifesto U-turn, at nearly the last minute of the campaign.
Tougher laws and police powers may fail to prevent low-technology attacks, such as the two van-and-knives assaults that occurred in London, and may well end up alienating British Muslims, he said.
The pledge appeared to be a clear contradiction of the Conservative manifesto promise to keep Britain in the European Convention on Human Rights until 2022.
Conservative sources said they would not withdraw from the ECHR but would seek opt-outs called "derogations" from certain aspects.
Mrs May has come under fire since the attack, with Labour leader Jeremy Corbyn calling for her resignation over her record on police cuts.
The former Tory Attorney General Dominic Grieve QC, who was removed from office in 2014 because of his opposition to party policy on scrapping the Human Rights Act, acknowledged that Tory plans would threaten the rule of law and rights across Europe.
Theresa May has said that she is willing to tear up human rights laws in order to combat the threat of terror attacks in the United Kingdom, in a last-ditch attempt to convince voters ahead of Thursday's election.
Home Secretary Amber Rudd said it was not for her or Mr Johnson to "say how that happened or what happened".
Liberal Democrat leader Tim Farron accused Mrs May of launching a "nuclear arms race" on terror laws and claimed she was "simply posturing about being tough on terror".