Hundreds of anti-monarchist demonstrators were among the thousands prevented from reaching the banks of the Thames by security staff as the jubilee flotilla passed by.
While the planned protest went ahead inside a designated zone for protesters, others denied entry due to overcrowding continued to protest on the other side of the fence, mingling with revellers making their way to Tower Bridge.
Insults were traded between royalists and anti-monarchists as demonstrators from the campaign group Republic chanted slogans and held banners reading: "Make Monarchy history" and "9,500 nurses or one Queen?"
"Some of these people have not thought about what they are celebrating, they are just having a party. Hopefully, they will think about the situation," said Finn Greig, 28, a Hackney youth worker, as a passing woman brandishing a Union Flag shouted: "disgraceful". He added: "I want to show I'm against the monarchy. It's about freedom of speech."
He was one of the visitors to the banks of the Thames who expressed frustration at not being able to get within sight of the river as security guards on private land closed the gates, as the flotilla set off towards Tower Bridge, saying capacity had been reached.
There was little confrontation between the two rival groups as the demonstration went on but some monarchists did express anger as they passed, answered with shouts of "sheep" from the Republic supporters.
Many revellers made their way to Tower Bridge, where a big screen had been set up to show the flotilla. Elaine Percy, 66, travelled from Cape Town to be with family. She said she could still remember the Queen's coronation.
She called the demonstrators "killjoys" but said they were brave for sticking to their protest in the face of thousands of monarchists.