Over half a million spectators are expected in central London for the start of the Tour de France, the world's biggest annual sporting event.
Hopes are high that a Briton may pull on the coveted yellow jersey after the 7.9km (five-mile) Prologue time trial around Westminster. Some 189 riders will power off from Whitehall, past landmarks including the Houses of Parliament, Buckingham Palace and Hyde Park before finishing on The Mall.
Eighteen giant TV screens will be erected around the Prologue route to allow spectators to follow the riders, who will set off one at a time from 3.00pm.
Each will hurtle around the course at speeds of up to 60km/h.
British speed rivals Bradley Wiggins, a Londoner who took gold in the 4km individual pursuit at the 2004 Athens Olympics, and David Millar, the last British rider to wear the yellow jersey in 2000, are hotly tipped for the Prologue. Millar, 30, who has won three Tour stages, served a two-year doping ban for EPO use.
Policing of the event has been reviewed in the wake of last week's terror attacks, which sees Britain on a "severe" security alert - its second highest security threat level.
Today is also the second anniversary of the July 7th attacks. Strong and "appropriate" policing will be in place, Scotland Yard said.
This is the first time the Tour has started in England, but the third time it has passed through in its 104-year history.
And about three million people packed the roadsides along England's south coast in 1994 when the Tour last visited Britain to mark the opening of the Channel Tunnel.
The event is watched by as many as 15 million people on the roadside every year with an estimated two billion watching on television over the three weeks.