According to the organisers of the protest around 60 per cent of those taking to the streets will be demo virgins - a fact that only serves to underline just how singularly Tony Blair has failed, to date, to win the PR battle for support of US-led action in Iraq.
This massive public veto, plus the fractures that have appeared in the NATO alliance, throw into stark relief how time is running out not only for Saddam Hussein but also for the Number 10 strategic communications unit.
There has been much talk on both sides of the Altantic of the need to ' get the message across'. And the frantic round of recent activity - including Blair's willingness to submit to an on-screen mauling by Jeremy Paxman in a bid to appeal directly to the electorate - indicates that Number 10 still nurtures hope that given the right packaging the public can be convinced. But based on current evidence this is a communications battle that cannot be won.
Even the rapid escalation of terror alert stories - including front-page images of tanks at Heathrow - have failed to secure a sufficient level of 'emotional' buy-in. Concern hasn't prevented the public rationally examining and rejecting the argument that Hussein presents an immediate threat or is linked with Al Qa'eda. It has not helped that part of the evidence put forward has itself been discredited, with the Government's dossier on Iraq now due to be cross-examined by the Cabinet Office's newly appointed review team.
The best that has been achieved to date is a state of genuine bewilderment and a lurking suspicion that there must be more to the available evidence than meets the eye for the UK Government to commit troops and funds to this extent.
But in this day and age this is not enough. The democratisation of information over the last decade and a resultant sense of personal empowerment has fundamentally altered public expectations of accountability.
Society is now just too information rich for such a blatant information gap to pass unnoticed, without comment and unchallenged. In the absence of hard facts, at the very least the public want the third-party endorsement of the UN. Without this, the argument falls into a classic credibility gap. The entire scenario reads like a press release without a news hook.
Whether or not you support the call to arms, from a professional standpoint this has got to be the PR brief from hell.