Ministers will be "gambling with the safety of the British people" if they press ahead with cuts to police budgets, Jeremy Corbyn will warn, amid fears forces could not deal with a Paris-style attack.
The Labour leader called for assurances that there would be "no reduction in numbers, essential equipment or helicopter support" as George Osborne finalises his Whitehall spending review.
Home Secretary Theresa May is among cabinet colleagues yet to agree a deal with the Chancellor, just days before he is due to unveil the latest round of deep spending cuts in the Autumn Statement.
A private letter to Mrs May from senior officers warned that a reduction in officer numbers would "severely impact" on their ability to respond to terrorism.
Mr Corbyn - making a speech postponed in the immediate aftermath of the Paris atrocities - will suggest the Government risked failing in its duty to keep people safe on the streets.
"Following discussions with Andy Burnham, we want to make this very clear: after Paris, there must be no cuts in the police front line," he will tell activists in the South West.
"That means no reduction in numbers, essential equipment or helicopter support. To press ahead with these cuts would be gambling with the safety of the British people."
He will say: "What's pro-British about ministers whose police cuts are so severe that, as senior officers have warned, they are expected to 'reduce very significantly' the ability to respond to a Paris-style attack?
"By pressing ahead with these cuts, the Government is failing in its most basic duty: to protect our citizens.
"They must be halted."
Mr Corbyn will say the UK's security has also been put at risk because of its foreign policy, putting itself "at the centre of a succession of disastrous wars that have brought devastation to large parts of the wider Middle East."
"They have increased, not diminished, the threats to our own national security in the process."
The Labour leader, who is firmly opposed to Britain joining allied air strikes against self-styled Islamic State (IS) extremists in Syria, is under pressure to allow Labour MPs a free vote on the issue as many appear ready to rebel on the issue.
He will say there needs to be "a different kind of foreign policy - based on a new and more independent relationship with the rest of the world, a relationship where war is a last resort".
Rejecting David Cameron's claim he has a "Britain-hating ideology", he will ask: "What's pro-British about a Government that slashes support for serving soldiers and military veterans?
"How is it patriotic to take money from the poorest, from working families, and hand control of your country to a super-rich elite? What's pro-British about a Government that slashes support for serving soldiers and military veterans?
"Labour will take no lectures in patriotism from the Conservatives, the political wing of the hedge-funds, the bankers and the 1% elite."
Mr Corbyn - who won the leadership by a landslide but is attracting increasingly hostile criticism from Labour MPs - will make clear his determination to take more control of policy-making and will insist he can lead the party to power in 2020.
A "democratic revolution" would open up the process to the hundreds of thousands of new members and supporters who swept him to victory in September, he will suggest.
In a clear riposte to critics warning of a left-wing takeover, he will say: "That's not about fighting sectarian battles or settling political scores.
"It's about being open to the people we seek to represent, giving them a voice through our organisation and policy-making, and drawing members into political action.
"Why not give members the chance to take part in indicative online ballots on policy in between annual conferences - and give our grassroots members and supporters a real say?"
He will defy critics of his left-wing stance and insist he has the "viable and credible alternative" needed to attract sufficient support to oust the Conservative government at the next general election.
"The Tories won in May on their lowest ever share of the vote to deliver a parliamentary majority, just 37% of those who voted and less than a quarter of those eligible. That's no landslide in anyone's book.
"But Labour failed to win back the economic credibility lost in the financial crash of 2008 or convince potential supporters we offered a genuine alternative.
"If we focus everything on the interests, aspirations and needs of middle and lower income voters, if we demonstrate we've got a viable and credible alternative to the Government's credit-fuelled, insecure, two-tier economy, I'm convinced we can build a coalition of electoral support that can beat the Tories in five years' time."?