Anti-monarchy protesters are preparing to stage a "loud, bold and provocative" demonstration during the Diamond Jubilee River Pageant.
Campaign group Republic said supporters from across the country will descend on the capital for the "biggest and boldest anti-monarchy protest of modern times". It will take place near Tower Bridge, on the south bank of the Thames by City Hall, close to the position the Royal Family will watch the passing boats.
Republic said it expected "hundreds of anti-monarchists" to join the protest, including representatives of the group's branches in Wales and Scotland, as well as a delegation from the Swedish republican movement.
Protesters said talks have been held with Metropolitan Police and reassured families intending to travel to Tower Bridge to watch the pageant that the protest will be peaceful.
They will hold placards and banners saying "citizen not subject", "power to the people" and "democracy not monarchy". Guest speakers at the event include human rights campaigner Peter Tatchell and columnist Joan Smith.
Republic's chief executive Graham Smith said: "Our message is to the wider public that there is an active and growing movement they can join rather than sitting and watching on TV. We are planning a peaceful protest and many families will be joining us.
"We can't wait for Sunday. Our loud, bold and provocative protest will be a major step forward for the republican movement and an opportunity to kick-start a real debate about the future of the monarchy.
"The royals spend most of their lives shielded from criticism; this protest will give them a rare glimpse of the strength of republican sentiment in Britain today.
"The hereditary system is offensive to all the democratic values this country has fought for in the past. The jubilee represents a celebration of everything we, as republicans, oppose. It is our right, and duty, to challenge it and promote the alternative.
"The heightened palace PR campaign of the last 18 months has already backfired. People are sick and tired of being told they must celebrate 60 years of one very privileged, very remote and very uninspiring head of state. The idea that the monarchy's future is any safer as a result of the jubilee is completely laughable."