The event, expected to attract around 2,000 people and organised by the Mental Health Alliance, is the centre-piece of a media relations and public affairs push calling for a rethink of the Government's draft Mental Health Bill.
The alliance claims the Bill will lead to an increased use of compulsory treatment and detention of those with mental health issues, to the extent that it could contravene the Human Rights Act.
Paul Corry, media relations manager of the charity Rethink, which is part of the alliance, said: 'The aim is to present the issues to MPs and Lords so that pressure can be put on ministers to delay the bill.
It is not acceptable in its present form.'
If this fails and the Bill is included in this year's Queen's speech, Corry has pledged that the alliance's member groups will target each stage of its transition through the parliamentary process. 'There will be line by line challenges,' he added.
A key part of the media campaign is to alter the focus of coverage away from public safety angles to human interest features about the lives of those with mental health issues. The tabloid press is a particular target for this strategy
Concerns by alliance members over the public's lack of awareness of mental health issues forced the cancellation of a march to Parliament planned for 14 September. It was felt negative coverage of the issue relating to the deaths of Holly Wells and Jessica Chapman in Soham would put the safety of those protesting at risk.
The alliance, whose members and associate members include SANE, the Royal College of Psychiatrists and the Law Society, was formed in 1999 when changes in the law were first mooted.