Jeremy Corbyn attempted to flank Theresa May and the Conservatives on the right by promising to boost police numbers and funding for the security services, in a speech in central London on Friday 26 May.
The Labour leader's address came as the major parties resumed campaigning in the wake of the Manchester suicide bombing, which left at least 22 dead and 59 injured.
Corbyn, a life-long anti-war campaigner, partly blamed Britain's military interventions abroad for the terrorist threat.
"Many experts, including professionals in our intelligence and security services have pointed to the connections between wars our government has supported or fought in other countries, such as Libya, and terrorism here at home," he said.
"That assessment in no way reduces the guilt of those who attack our children. Those terrorists will forever be reviled and implacably held to account for their actions.
But an informed understanding of the causes of terrorism is an essential part of an effective response that will protect the security of our people, that fights rather than fuels terrorism."
But the left-winger also promised to put more police on the streets. "And if the security services need more resources to keep track of those who wish to murder and maim, then they should get them," Corbyn promised.
He added: "Over the past fifteen years or so, a sub-culture of often suicidal violence has developed amongst a tiny minority of, mainly young, men, falsely drawing authority from Islamic beliefs and often nurtured in a prison system in urgent need of resources and reform.
"And no rationale based on the actions of any government can remotely excuse, or even adequately explain, outrages like this week's massacre."
The remarks came as Labour cut the Tories' lead in the opinion polls to just five points, according to YouGov.
But the survey, of more than 2,000 people between 24 and 25 May, found that just 18% voters thought Labour would best handle defence and security issues, whilst 41% of respondents said the Conservatives were best placed.
On law and order, the figures are better for Corbyn, with 21% of people backing Labour and 38% of respondents picking the Tories.
The Conservatives have attacked Labour on the issue throughout the campaign, with past comments and actions of Corbyn and Shadow Chancellor John McDonnell being raised.
McDonnell, for example, once backed a campaign calling for Mi5 and armed police to be disbanded. He was also forced to apologise for a 2003 speech in which he said Republican terrorists the IRA should be honoured for their bombing campaign.
"If I gave offence – and I clearly have – from the bottom of my heart I apologise," he told a BBC Question Time audience. The general election is now under two weeks away on Thursday 8 June.