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The tour's director believes double the official figure of spectators watched the event in the county.
Speaking after stage two finished in Sheffield, Christian Prudhomme said the huge crowds seen for the second day out on the route were ``unbelievable, incredible, amazing, astonishing.''
The official police estimate for the number of people who lined the route of the Grand Depart in Yorkshire this weekend is 2.5 million.
But Mr Prudhomme said: ``I don't know. Perhaps in two days, perhaps five million, I don't know. It was unbelievable, unbelievable.''
He said: ``It will stay as the grandest Grand Depart ever.
``Yesterday I was very, very impressed, I was delighted. But today, it was unbelievable, incredible, amazing, astonishing.''
Today's 201km stage from York to Sheffield saw more scenes of crowds packing the roadsides following yesterday's events in Leeds, Harrogate and the Yorkshire Dales.
Perhaps the most dramatic were on the bleak moors of Holme Moss, above Huddersfield, where an estimated 60,000 cheered as the 197 remaining riders fought their way to the summit.
But there were similar scenes in other, normally quiet corners of Yorkshire, especially on the climbs which made today's stage one of the toughest opening days in Tour history.
On the roads near Bradfield and above Oughtibridge, north of Sheffield, and at Cragg Vale, near Halifax, thousands of people crammed on to the narrow lanes urging the riders on, only inches from their handlebars.
Today's stage started without British favourite Mark Cavendish who was forced out of the race following his dramatic crash yesterday.
``I'm absolutely devastated,'' Cavendish said before the start in York.
``I was in pain last night. I held a bit of optimism that it was maybe just swelling and would go down overnight, but it's actually worse this morning.''
Cavendish came to grief almost within sight of the finish line in Harrogate - his mother's home town - giving yesterday's opening stage a dramatic climax.
This morning, thousands of people waved the riders off at York racecourse before the peloton made its way past the city's Minster and other famous landmarks.
A Lancaster bomber and a Spitfire saluted the arrival of the Tour with a flypast over the city.
Race organisers estimated that 100,000 people watched the race in York today.
By the time the riders reached the first of the stage's many challenging ascents, at Blubberhouses, between Harrogate and Skipton, the riders knew they were going to get the same frenzied reception they got when more than a million people turned out to watch yesterday's first stage.
At Holme Moss vast numbers of people stretched along the steep road and up the hillside on the northern side.
Many of those in the prime spots near the summit had camped nearby to get the best positions.
British Team Sky rider Geraint Thomas said: ``It was unbelievable at times. Going up Holme Moss, I had goosebumps. It was amazing.''
Sir Rodney Walker, chair of TdFHUB2014 Ltd, which organised the UK Grand Depart, said: ``Around 2.5 million spectators lined the route over two days and revelled in being part of history. The passion of the crowds in Yorkshire has really made this a weekend to remember.
``There has been a huge amount of planning and hard work from all the partners involved to ensure the first two stages were a success.
``We have once again showcased how the UK can deliver amazing events and a world-wide audience has seen the best of Yorkshire, and the best of the UK.
``I have no doubts the benefits of hosting the Tour de France in the UK will be felt both in the short and long term.''