Savings equivalent to 50p in every £100 the government spends need to be found by 2020, the Chancellor said.
But the cuts are “not a huge amount in the scheme of things”, Mr Osborne told BBC One’s Andrew Marr Show.
He said: “My message in this Budget is that the world is a more uncertain place than at any time since the financial crisis and we need to act now so we don’t pay later.”
Mr Osborne appeared to indicate that the Government may hold back from fuel duty rises in Wednesday’s Budget. The Chancellor has come under pressure from Tory backbenchers to resist raising the tax. Around 640,000 people will be affected over the next five years by changes to Personal Independence Payments.
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But Mr Osborne denied claims that he was hitting the most vulnerable through cuts to disability benefits.
Mr Osborne also defended the £130 million tax deal Google reached with HMRC, which was criticised by MPs for being “disproportionately small’’.
The Budget will include a clampdown on a tax loophole widely exploited to disguise employees as freelancers.
Civil servants, along with institutions like the BBC and the Bank of England, will be hit by rule changes to stop earners gaining tax advantages by being “off the books”.
Around 20,000 public sector workers are avoiding an average of more than £3,500 a year in income tax and National Insurance contributions under the current system, according to the government.
As Mr Osborne tried to put the finishing touches to his Budget, his concentration was disrupted by “noisy” filming outside the Treasury by the new Top Gear team, he revealed on Twitter
The Chancellor tweeted host Chris Evans pleading with him to “keep it down”.
He said: “Trying to write my Budget, despite noisy episode of BBC Top Gear being filmed on Horseguards Parade. Keep it down please Chris Evans.”