By Agence France-Presse
The march on parliament is set to be a noisy and festive affair uniting tens of thousands in the seemingly hopeless task of convincing Prime Minister Theresa May to hold a second Brexit referendum.
Even dogs are coming.
Organisers of the so-called People's Vote March are asking people to dress up their favourite pets in costumes so that they could bark along the scenic route through the historic heart of London.
The message itself will be serious: the Brexit its supporters promised ahead of the June 2016 referendum that set the divorce in motion looks nothing like the one being negotiated today.
Campaigners say they would have voted differently had they known the true costs involved.
"It's time the great silent and not-so-silent majority made their feelings known about Brexit," anti-Brexit activist Gina Miller wrote in The Independent newspaper.
"We must shout with one voice: 'Not in my name!'"
The pro-EU newspaper's online petition demanding a binding vote on any deal agreed before the March deadline has been signed nearly a million times.
The British premier has made it abundantly clear that she has no intention of allowing a do-over.
"They now want a second referendum to go back to the British people and say 'Oh, we're terribly sorry -- we think you've got it wrong,'" May told parliament on Wednesday.
"There'll be no second referendum. The people voted and this government will deliver on it."
Yet what May's government is actually delivering is not entirely clear.
The alternatives facing Britain at the moment range between a clean break without any agreement and one in which little changes except for London losing its voice in the EU.
Neither choice is appealing. Recriminations over how Britain got here are leaving May looking increasingly isolated and weak.
And EU leaders such as French President Emmanuel Macron are openly wondering if a second British vote might yet make the mess go away.
Polls show support for a second referendum evenly split -- the same as with Brexit itself.
The 2016 referendum was backed by 52 percent of voters on turnout of 72 percent.
But some think MPs may rally around the idea once they see what Britain might be forced to sign up to -- and they must approve -- at the last moment to avert complete chaos.
The last big march on parliament demanding a second vote in June saw an estimated 100,000 gather on and around the picturesque Parliament Square facing Westminster Palace.
Organisers say this one will be bigger and involve supporters of all major parties from every corner of Britain -- and beyond.
"More than 150 coaches will bring people from towns and cities from every region and nation," they said in a statement.
The speaker's list is headlined by London Mayor Sadiq Khan while Gary Lineker -- a retired football star familiar to just about every Briton -- will pipe in a video message of support.
And groups representing several million EU citizens in the UK will unite with their British counterparts in Europe to march under a single banner from Hyde Park.
The route itself will twist from the giant park to the central Trafalgar Square before reaching parliament.