Jeremy Corbyn has criticised Theresa May's record on dealing with the terror threat and accused her of denying resources to the police and security services.
He also sought to end the earlier controversy over his support for police use of "shoot-to-kill" tactics against armed attackers, by stating that he backed the "full authority for the police to use whatever force is necessary to protect and save life as they did last night, as they did in Westminster in March".
In a speech in Carlisle a day after the terror attack at London Bridge, the Labour leader vowed to take "whatever action is necessary and effective" to preserve public safety. He also warned the Government it could not "protect the public on the cheap".
The Prime Minister cut police manpower by 20,000 despite warnings that this would undermine safety, said Mr Corbyn, who has promised to recruit an additional 10,000 officers and 1,000 security service agents if he wins power on June 8.
Earlier in the day, Mrs May set out a four-pronged strategy to tackle terror by countering radical ideology; clamping down on online extremism; preventing the growth of segregated communities; and giving extra powers to police, security agencies and courts.
The PM's comments sparked complaints from Labour that she was getting involved in political debate on a day when the parties had agreed to halt election campaiging until the evening.
Mr Corbyn backed Mrs May's insistence that the General Election must go ahead on June 8.
He said the attacks at Manchester Arena and London Bridge had turned Thursday's vote into a "struggle between terrorism and democracy itself".
He urged voters to "resist Islamophobia and division and turn out on June 8 united in our determination to show our democracy is strong".
Speaking as the brief pause in campaigning came to an end, Mr Corbyn said: "Our priority must be public safety and I will take whatever action is necessary and effective to protect the security of our people and our country.
"That includes full authority for the police to use whatever force is necessary to protect and save life as they did last night, as they did in Westminster in March."
Criticising Mrs May's record, he said: "You cannot protect the public on the cheap. The police and security services must get the resources they need, not 20,000 police cuts.
"Theresa May was warned by the Police Federation but she accused them of 'crying wolf'.
"We will recruit another 10,000 new police officers, including more armed police, as well as 1,000 more security services staff to support our communities and help keep us safe."
After Mrs May said that combating terrorism would require "difficult conversations" with Muslim communities in the UK, Mr Corbyn said that the PM must also be ready to have difficult discussions with close ally and major arms customer Saudi Arabia about terror funding.
He cited the delayed publication of an investigation commissioned by David Cameron into the foreign funding of extremist Islamist groups, which is reported to focus on the Gulf kingdom.
"We do need to have some difficult conversations, tarting with Saudi Arabia and other Gulf states that have funded and fuelled extremist ideology," said Mr Corbyn.
"It is no good Theresa May suppressing a report into the foreign funding of extremist groups. We have to get serious about cutting off the funding to these terror networks, including Isis, here and in the Middle East."
Mr Corbyn said that the terrorists' aim was "plainly to derail our democracy and disrupt or even halt this election".
"The mass murderers who brought terror to our streets in London and Manchester want our election to be halted," he said.
"They want democracy halted. They want their violence to overwhelm our right to vote in a fair and peaceful election and to go about our lives freely. "That is why it would be completely wrong to postpone next Thursday's vote, or to suspend our campaigning any longer."