But inside were 23 bemused anti-monarchist protesters, flanked by more than 40 officers, who for up to two-and-a-half hours took them on a forced tour of the capital.
Yesterday the magical mystery tour ended with the Metropolitan police's attempt at running a tourist bus costing the force £80,000 and a grovelling apology.
The Met paid the damages to the 23 anti-monarchist protesters who were plucked out of a pub on the Queen's golden jubilee.
Officers had commandeered the Number 11 bus after flagging it down, then turned it into a prison on wheels, dropping off the protesters at five police stations across east, north and south London.
Scotland Yard yesterday hailed their officers' "ingenuity". The protesters called them "Keystone cops".
Faced with legal action, the police admitted that they had no evidence to arrest the demonstrators and have apologised for detaining them. No one was charged.
It all began on June 4 2002, the day of the Queen's golden jubilee. More than 14,000 police were on duty as tens of thousands of people thronged London.
Among them was a small group of anti-monarchist protesters. They had held a peaceful demonstration shouting anti-royal slogans at Tower Hill, before retiring to the Goodman's Field pub in east London for a lunchtime pint.
Police arrested 19 anti-monarchists inside and four nearby, before placing them on the bus, which was stopped as it headed to Bow, east London.
A Scotland Yard spokeswoman said the bus was needed because of the number of people arrested and the scores of officers who accompanied them: "The officers used their ingenuity. They saw a passing bus, spoke with the driver, and it was agreed that they could use it as a mode of transportation.
"Officers apologised for the inconvenience and members of the public made alternative travel arrangements."
The 23 protesters each received £3,500 as the Met settled their lawsuit out of court.
In the letter of apology, the Met wrote: "I am writing to apologise on behalf of the Metropolitan police for the fact that you were arrested and detained for some hours.
"It is accepted that there was insufficient evidence to justify your arrest on this occasion and that you should have not been arrested and detained."
Trevor Bark, 37, said he was just settling into his second pint of Spitfire bitter, when he saw through the pub window that police in protective clothing had ringed the pub: "They stopped people leaving. When they had enough police they came into the pub and nicked everyone."
Officers quizzed the anti-monarchists one by one about whether they intended to cause a breach of the peace. All answered no, but were arrested anyway.
According to the arrest reports seen by the Guardian, the reason given for the detentions by officers was "intelligence from Chief Inspector Page that the group inside may cause trouble".
The arrest reports said arresting officers were told by their superiors to detain the anti-monarchists whatever answers to questions were given. The arrest report from one constable reads that he was told "all answers are to be disbelieved".
Mr Bark, a PhD student, said: "They marched us outside, each demonstrator had two police officers on them, then they searched us all.
"I saw the bus, and was taken upstairs near the front." He said he was not handcuffed, but others were.
"It was surreal being put on the bus, it was Keystone cops. The bus ran red lights and drove fast," Mr Bark alleged.
"I resented every second of it; we know they hate us, but you should be able to protest." He was part of the group dropped off first and taken to Bishopsgate police station, where he says he offered no resistance: "When you are in the belly of the beast you don't want to kick up a fuss."
After Bishopsgate, the bus drove to Shoreditch in east London, then dropped off more protesters at Islington in the north of the capital, before trundling down south over the river Thames to Plumstead in south-east London. The last protesters were dropped off at Walworth Road police station in south London.
Mr Bark said the apology did not go far enough: "They are not holding their senior officers accountable. They're not apologising for breaking the law and making the wrong decision."
The solicitor for the protesters, Mark Scott, said: "It's quite a serious case as the police were seeking to restrict lawful protest and there was no lawful basis for these arrests. It shows the police overstepping the mark."
A spokeswoman for Transport for London said: "Obviously, we do not want to disrupt passenger services but if it's something that will protect people, we will cooperate with the police."
In a statement, the Met said: "On this occasion the arresting officers transported prisoners to the custody suite on a public bus.
"This was deemed to be the best course of action due to the fact that such a large number of people needed urgent transportation."
After nearly seven hours in custody Mr Bark was released and made his own way home.
He took the bus.
