Anti-monarchy protesters faced renditions of the national anthem, jeers and boos as they staged a demonstration at the Queen's Diamond Jubilee river pageant.
Dozens of supporters of campaign group Republic gathered on the banks of the Thames near Tower Bridge, ahead of the arrival of the 1,000-strong flotilla led by the Queen.
The protesters waved placards and banners saying "citizen not subject", "power to the people" and "democracy not monarchy" as they called for the abolition of the monarchy.
But they faced boos and jeers from sections of the crowd who had waited for hours in the rain to watch the river pageant, while some royal supporters began singing God Save The Queen.
Graham Smith, chief executive of Republic, said the protest was held at the same time as the river pageant to "raise the profile" of the republican movement.
He said: "We're opposed to celebrating hereditary privilege and power. The message is to the millions of people who oppose the monarchy that there is a movement you can support.
"The whole country isn't in love with the monarchy. It's a minority interest. We'll keep going until the monarchy is abolished."
Protesters said talks have been held with the Metropolitan police and reassured families intending to travel to Tower Bridge to watch the river pageant that the protest would be peaceful.
Mr Smith added: "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."