And despite the underlying seriousness of the issue, campaigners plan to turn the march, from Hyde Park to Trafalgar Square, into more of a celebration than a demonstration.
Sarah Russell, a mature student from Leeds, is one of hundreds who rang our special march information line: "We have organised our own coach and are hoping like mad that the weather is going to hold up because we are determined to have a great time. It will be just like one big party to be with so many like-minded campaigners ... saying it loud and proud."
Yesterday columnist Charles Glass, writing in the Evening Standard, urged Londoners to support the march. "I call on everyone who came out for the countryside to return to the streets in the same cause: freedom. From Marble Arch to Trafalgar Square, thousands of men, women and children will parade to show the Government that those who smoke marijuana should not be sent to prison for it. Smokers are not a criminal minority, they are just ordinary people," he wrote.
However, unlike the Countryside Rally supporters, some of the cannabis marchers might look more than a little bleary eyed. "A large number of flyers have been distributed around the London club scene and quite a number of enthusiasts have said they will go straight to Hyde Park from their all-night parties," said a volunteer worker for Release, the drugs charity.
Labour MP Paul Flynn who is trying to get cross-party support for drug- law reform believes the march is already a success. "Notwithstanding an earthquake or flood this march has already achieved a great deal," said Mr Flynn who will speak at the Trafalgar Square rally. "The extraordinary level of media interest that has already been created by this march means that every household in Britain will have the opportunity of discussing the subject of cannabis and the law this weekend. As things stand, the three major parties are conspiring to stifle debate on this subject. There is one simple message to get across and that is decriminalisation works, prohibition does not work."
Supporters are not just rallying in Britain. This week news of the Independent on Sunday march went international, creating a buzz of home and overseas media activity. Canadian television is in London to cover the march and an Italian radio station is going to transmit coverage of the whole of tomorrow's event live. There are even plans to broadcast the march on the Internet. Preliminary television interviews with some of the speakers have already been syndicated internationally, and Australian radio and the BBC World Service have also featured the march in their news coverage.
"The message seems to have got out into the wider world this week. We have had dozens of calls from Europe. One group of individuals from Paris rang to ask which was the nearest tube for Hyde Park as they were coming over on Eurostar for the day," said Chris Brown who has been working on the march information phone line this week.
Other groups are expected from Holland, Belgium and France. A strong delegation is expected from Rome to support Marco Pannella the founder of the Italian Radical Party and veteran campaigner for drug-law reform. After the march Mr Pannella is planning his own press conference.
Others will be going to the special after-march party organised by Hempology at the Cloud Nine club in London SE1. Doors open at 10 pm and will feature a guest appearance of Trafalgar Square speaker Howard Marks who is to DJ into the early hours.
CAMPAIGN WHO'S WHO
THE CAMPAIGN to decriminalise cannabis has won backing from some of Britain's liveliest minds. Anita Roddick, founder of the Body Shop ethical cosmetics chain, spoke movingly at the seminar on decriminalisation organised by The Independent on Sunday last December.
Recently, she announced her chain of shops would introduce a range of beauty products based on hemp-seed extracts - drawing the wrath of former Tory Home Office Minister Anne Widdecombe.
Richard Branson, the entrepreneur businessman, has lent his name and backing to the campaign. Sir Paul McCartney, while supportive, has preferred to play a low-profile role in the campaign.
The visual arts have been represented by film directors Mike Leigh and Peter Greenaway. Fay Weldon, A N Wilson and Nick Hornby are among a host of leading writers to back the campaign. Prominent medical supporters include Dr Philip Robson, consultant psychiatrist at the Warneford Hospital, and Professor Steven Rose, director of the Brain and Behaviour Research Group at the Open University.
For march information ring: 0181-964 2692.