ALPE D’HUEZ, France >> One of cycling’s big attractions is that fans don’t have to pay to watch — and they can get as close as they want to the riders.

The sport paid for that unlimited access today when unruly spectators interrupted the most iconic climb of the Tour de France.

With fans pressing too close to an elite group of riders at the conclusion of the 21 hairpin bends up to Alpe d’Huez, top contender Vincenzo Nibali crashed into a police motorbike and later withdrew from the race, four-time champion Chris Froome received some undesired contact on his back, and yellow jersey holder Geraint Thomas was booed on the podium.

“If people don’t like Sky and want to boo, that’s fine. Boo all you like, but don’t affect the race,” Thomas said. “Don’t touch the riders. Don’t spit at us. Voice your opinions all you want but let us do the racing.”

Attitudes toward Sky soured when Froome was involved in an asthma drug case stemming from last year’s Spanish Vuelta — even though he was cleared of wrongdoing just days before the Tour started.

Froome is attempting to match the record of five Tour victories shared by Jacques Anquetil, Eddy Merckx, Bernard Hinault and Miguel Indurain.

Thomas sprinted away from Sky teammate Froome and a couple of other riders to win Stage 12, becoming the first rider in Tour history to win atop Huez in the yellow jersey.

Lance Armstrong won an individual time trial up Huez in 2004 while wearing yellow but that victory was later stripped for doping.

“Unbelievable. Not in my wildest dreams did I ever think I would win up here,” Thomas said. “It’s one of those things that’s going to stay with me for the rest of my life.”

But Thomas acknowledged that Nibali’s crash took some of the luster away from his victory.

Thomas rode over Nibali’s rear wheel — when the Italian went down — but managed to stay upright. Nibali, the 2014 champion got back up but finished seventh in the stage, then was taken to the local hospital for medical tests for a suspected back injury.

“He should have been fighting for victory,” Thomas said of Nibali. “It’s a bit of a downer on the day.”

Nibali said: “The road became narrower and there were no barriers. There were two police motorbikes. When Froome accelerated, I followed him, I was feeling good. Then we slowed down and I hit the ground.”

Nibali was diagnosed with a fractured vertebra and the Bahrain-Merida rider later tweeted that he was withdrawing from the Tour.

Tom Dumoulin crossed second, two seconds behind Thomas, and Romain Bardet came third, three seconds back.

Froome, who finished fourth, four seconds behind, refused to talk to reporters afterward. A fan who allegedly hit him during the stage was put in handcuffs by French police.

“You expect professional athletes to play sport and entertain without being touched or impacted on by the crowd,” Sky general manager Dave Brailsford said. “Part of the joy of our sport is how close the crowd gets. But we have to bear in mind that if it impacts on the race, as it did with Nibali today, then that’s too much.”

Thomas extended his lead over Froome in the overall standings to 1 minute, 39 seconds. He’ll likely hold on to the yellow jersey for at least several more days as the Tour returns to flatter roads for Stages 13 and 14 before the climbs resume in the Massif Central and the Pyrenees.

With questions over which rider Sky is backing for victory, it was a bold demonstration of strength by Thomas, who has been Froome’s loyal lieutenant for years.

“In my eyes Froomey is still our leader,” Thomas said. “I’m just going to enjoy it now.”

Dumoulin is third overall, 1:50 behind, while Nibali recovered to hold on to fourth, 2:37 back, before withdrawing.

The last and most feared of the three stages in the Alps this year, the 175.5-kilometer (109-mile) leg began in Bourg-Saint-Maurice and took the peloton over three grueling, beyond-category climbs.

Fans lined every corner of the climb to Huez, many of them amateur cyclists who tested their legs on the ascent a few hours before the professionals, while others had camped out for days.

As usual there were many rowdy fans on the “Dutch corner” located two-thirds of the way up.

Riders had to navigate their way through smoke in a variety of colors — red, blue and orange.

Before Huez, riders had already scaled the lengthy ascents to Col de la Madeleine and Col de la Croix-de-Fer (Pass of the Iron Cross) plus the shorter but spectacular Lacets de Montvernier — a miniature Huez featuring a photogenic series of 18 switchbacks.

Part of an early breakaway group, Dutch rider Steven Kruijswijk launched a solo attack up the Col de la Croix-de-Fer and crossed the summit above the tree-line with a six-minute lead over the peloton. But he was caught by Froome on the way up to Huez with 3.5 kilometers remaining.

“We didn’t expect Kruijswijk to attack so hard,” Thomas said. “They really took the race to us.”

Froome had attacked with four kilometers to go but couldn’t quite drop Thomas, Dumoulin and Bardet.

Rigoberto Uran, last year’s runner-up, withdrew before the stage began, having failed to recover from a crash on the cobblestones in Stage 9.

The mountainous route was a nightmare for muscular sprinters. Fernando Gaviria and Dylan Groenewegen, who each won two stages each in this year’s race, withdrew midway through, as did Andre Greipel.

The three-week race ends July 29 in Paris.