Dozens of demonstrators, some of whom were drinking, stopped the vehicle from moving in Trafalgar Square before passengers were ushered away.
The group pushed “Britain Loves Trump” posters onto the vehicle’s windscreen.
But protesters appeared on the brink of fighting among themselves as some demanded the bus be let through while others rattled its sides.
Police were present throughout the incident but did not move in until reinforcements arrived. One liaison officer said they did not want to escalate the situation.
The Metropolitan Police’s events team tweeted: “There is now a protest in the road at Trafalgar Square, and consequently there is currently no vehicle movement around the Square.”
The bus driver appeared unperturbed throughout. When politely asking the demonstrators to move failed, she remained sat at the wheel rolling her eyes. She was allowed to drive on after approximately 30 minutes after police finally cleared the road.
Twelve people were arrested in total throughout the day for a range of public order offences, although it remained unclear if any were connected with the bus blockade.
In once incident a union leader is said to have been attacked at a nearby pub. Steve Hedley, senior assistant general secretary of the RMT, had spoken at a separate counter-protest in Jubilee Gardens and said he was later the victim of an “unprovoked” assault in the Westminster Arms.
A Twitter post showed him covered in blood, though the identity or affiliation of his assailants was not clear.
Earlier in Whitehall anti-Islam chants and pro-Trump cheering were heard as a series of speakers addressed the combined “Free Tommy Robinson” and “Welcome Donald Trump” rallies.
An inflatable pig with a mask of London mayor Sadiq Khan attached to it was passed about the 6,000-strong crowd. Mr Khan has repeatedly clashed with Mr Trump on Twitter and in the press.
One banner labelled Angela Merkel, Germany’s chancellor, “Merc***”. Two fireworks were let off in the middle of the mass.
Ukip’s new leader Gerard Batten and controversial US congressman Paul Gosar were among those who spoke. Geert Wilders, the far-right Dutch politician, starred via a pre-recorded video message. In a 10-minute tirade, he appeared to suggest Robinson, who founded the English Defence League, attempt to become prime minister.
“Tommy does not deserve jail,” he said. “Tommy deserves a medal…You are the pride of this nation. Around the corner is Downing Street 10. And who lives there? Theresa May. But who deserves to live there? Tommy Robinson.”
He added: “Tommy, if you hear this, know this: I say we will never abandon you. We will not rest until you are a free man.”
To the side of the stage, one protester – a can of lager by his hand – snoozed under a tree.
An earlier march had largely failed to materialise when only 100 or so people turned up to Temple Place, near Embankment, to walk to Whitehall. The vast majority of protesters appeared to have spent the early afternoon, instead, outside Whitehall’s pubs soaking up the sun.
Robinson, real name Stephen Yaxley-Lennon, was jailed earlier this year for 13 months after admitting committing contempt of court. He had recorded a Facebook Live video which contained details of a court case subject to temporary reporting restrictions.
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