Five individuals were arrested by police after violence erupted on the streets of the capital.
Crowds of Tommy Robinson supporters blocked off a major road in central London in a tense stand-off with police.
Hundreds of demonstrators descended on the capital on Saturday afternoon, and blocked off the road around Trafalgar Square.
Five officers have reported injuries, after bottles, metal barriers and other objects were thrown.
Their injuries are not serious.
Rows of riot police blocked the gate down the Mall leading to Buckingham Palace where the Royal Family gathered on the balcony after celebrating Trooping the Colour just hours before.
The protest is the latest in what appears to be a bid to secure the former English Defence League (EDL) leader’s release from jail.
Supporters chanted “Free Tommy Robinson” and hurled missiles and smoke bombs at police.
Hundreds of supporters crowded on to an open-top Megabus sightseeing vehicle waving Union Jack flags, while one supporter posed on the roof in a Donald Trump mask.
By 6pm, officers had removed protesters from the bus and had kettled many of the remaining protesters on the traffic island overhead.
Other Robinson supporters crowded on to the steps of Nelson’s Column and continued to chant slogans.
A spokeswoman for megasightseeing.com said: “Our London sightseeing bus was on its normal route when it got caught up in the demonstrations.
“The bus was stormed by demonstrators and the driver and a small number of customers got off.
“The demonstrators have caused a significant amount of damage to the bus which meant it had to be towed away.
“We have reported this to the police and will help them with any investigations.”
Videos posted on social media appeared to show police being significantly outnumbered by demonstrators.
The Metropolitan Police have not yet responded to requests for comment about the protest.
Robinson was jailed for contempt of court after he used social media to broadcast details of a trial which is subject to blanket reporting restrictions.
Leeds Crown Court heard how Robinson filmed himself and people involved in the case, in footage that was watched around 250,000 times within hours of being posted on Facebook.
Robinson, who was listed by his real name Stephen Christopher Yaxley-Lennon on court documents, was sentenced to 13 months in jail on the same day as his arrest.
He was given 10 months in jail for contempt of court, and an additional three months for beaching a previous suspended sentence.