Olympic rings mounted on a barge pass under Tower Bridge during a promotional event on the River Thames in London, Feb 28, 2012.
With 150 days to go before the July 27 Olympic opening ceremony, Johnson watched from the riverside as a tug pulled a barge with the five coloured rings through the open Tower Bridge while schoolchildren waved flags and a steel band played.
The Rings on the River, standing 11 meters high and 25 meters wide, were due to travel down to the Thames barrier, past Canary Wharf and Greenwich, before returning to the 19th century bridge next to the Tower of London.
They then go into storage to re-emerge during Games' time as part of the 'Look and Feel' of London 2012.
"In Games time you are not only going to be able to watch them, but when they dock at various points you are going to be able to mount bicycles and churn your legs and by some miraculous procedure several jets of water will come up around those fantastic rings," beamed Johnson, as renowned for his love of cycling as for his boyish exuberance.
"The serious point is that, I suppose, with 150 days to go, it shows that London's preparations are very far advanced and that these Olympics are on time and on budget."
Britain's Olympics Minister Hugh Robertson said earlier that the country was likely to have money left over from the 9.3 billion pounds ($14.73 billion) in public funds earmarked for the Games.
"We're on time, we're on budget and in fact we've got about half a billion pounds left in the budget," said Johnson. "But it's absolutely vital that we work very hard now to deliver all the programs.
"Today we are also announcing a lot of our cultural events. There are going to be Shakespeare performances popping up across London, opera taking place in London's canals, all sorts of events and they are going to be free to everybody."
Olympic rings mounted on a barge are manoeuvred to pass under Tower Bridge during a promotional event on the River Thames in London, Feb 28, 2012.
The events will include London's biggest ever outdoor arts festival and free events to be held in every borough of the capital from July 21 to Sept 9.
There will be a floating opera inspired by Edward Lear's 'The Owl and the Pussy Cat' and, staged on the canals, a series of free events called "Secrets: Hidden London" to encourage visitors and residents to see the city in a different light.
"Wherever you are you will feel a part of the 2012 celebrations and experience a summer like no other in one of the most exciting cities on Earth," said Johnson.
Olympic rings, mounted on a barge, pass the Tower of London as they move east along the Thames river to celebrate 150 days until the games begin, in London, Feb 28, 2012.
Yeomen Warders, also known as Beefeaters, are seen through Olympic rings mounted on a barge, as it is positioned in front of the Tower of London on the River Thames in London, Feb 28, 2012.