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 on Sunday 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.
The bookmaker Coral has opened betting on a range of Jubilee-themed odds.
Punters can bet 12-1 that Prince Phillip is spotted asleep during the Thames Pageant, or 100-1 that Her Majesty's boat sinks.
The odds on a member of the Royal party falling in the river are the same as a swimmer interrupting the pageant: 33-1.
And for those who fancy The Queen's chances of reaching the age of 100, there are odds of 16-1 that she will be celebrate her Platinum Jubilee in February 2027.
The first Diamond Jubilee river rehearsal of the bell ringers on the Belfry barge has taken place on the Thames. The barge will lead the Queen's Jubilee flotilla and carries eight church bells which will peel out across London as the procession makes its way up the Thames.
Pushed along by a tug at about 6mph, the vessel will mark the start of the Pageant, leading the flotilla as it makes its way from Battersea Bridge to Tower Bridge.
The Prince of Wales, Duchess of Cornwall and Prince Harry are set to join the Queen and the Duke of Edinburgh on the lavishly decorated royal barge as part of the Diamond Jubilee celebrations.
The Duke and Duchess of Cambridge are to join the Queen on her royal barge during the River Thames pageant celebrating the Diamond Jubilee.
William and Kate will be at the Queen's side as receives the well wishes of up to a million spectators expected to line the river banks and open spaces.