Chris Froome has been pulled off his bike by police after a difficult 17th stage of the Tour de France which saw him fall further behind teammate Geraint Thomas in the race for the yellow jersey.

Team Sky told media there was a misunderstanding after the stage as Froome rode back down a hill to his bus.

When police ordered him to stop, he lost control.

Photos showed a policeman pulling Froome by the arm as his bike lay on the ground.

Sky said Froome was not injured.

Froome had put a raincoat over his racing uniform to keep warm, on a day when he cracked on the final climb to Col du Portet, drastically decreasing his chances of a record-tying fifth title.

Froome finished one minute, 35 seconds behind stage winner Nairo Quintana.

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Froome's Sky teammate Geraint Thomas kept hold of the yellow jersey, one minute and 59 seconds ahead of second-placed Tom Dumoulin.

The incident came a day after police used tear gas to disperse a farmers' protest that had blocked the road with bales of hay.

Froome was among a large group of riders whose eyes needed treatment due to the tear gas.

Froome has been a repeated target of fans in this Tour after he was cleared of doping five days before the race began.

He had been racing under the cloud of a potential ban for using twice the permitted level of salbutamol during his victory at the Spanish Vuelta in September.

He said he has been repeatedly spat at since the race started, and spectators have punched him and tried to make him fall off his bike.

Thomas now the favourite, Sagan crashes

Until recently, Thomas had insisted Froome was Team Sky's leader, yet that looks to have changed after the Tour de France's yellow jersey holder emerged as the British outfit's best, if not only, chance of winning the race.

Geraint Thomas now believes he is in the best possible position to win a first-ever Tour de France title. ( AP: Christophe Ena )

Dutchman Dumoulin leapfrogged defending champion Froome into second, after a decisive attack 2 kilometres from the finish.

Froome, 2:31 off the pace with three competitive stages left, said he would now "look after" Thomas, effectively conceding that his own hopes of success had been dashed.

The Welshman, who had already claimed two stage wins in the Alps, had once again looked the strongest of the main contenders.

He took third place behind Colombian Quintana and Ireland's Dan Martin, but gained time over Froome, Dumoulin and fourth-placed Primoz Roglic.

That prompted a change of tune from Thomas.

Asked who was the Team Sky leader, he replied: "I'm in a good position now.

"I'm not going to change my mental approach and take it day by day, keep doing the small things right."

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Meanwhile, Peter Sagan, the three-time defending world champion and three-time stage winner in this year's race, crashed on the descent from the Col de Val Louron, the second mountain of the day.

Sagan made it to the finish with his jersey torn and said he had only injured his backside.

"That was a hard crash but, fortunately, I haven't broken anything and I feel confident about the last four stages of Le Tour de France," the man currently wearing the sprinters' green jersey later posted on Facebook.

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Froome's struggles give Thomas confidence

Thomas, who has never previously been in a position to win a grand tour, is keeping his feet on the ground.

"As soon as you get carried away, it's when it goes downhill," the two-time Olympic track champion said.

Chris Froome fell further behind teammate Geraint Thomas, before he his altercation with police after the 17th stage. ( AP: Peter Dejong )

Thomas praised the work of his teammates after they controlled the pace of the race all day, preventing most of their rivals from attacking as they set a high tempo in front of the main pack.

"It was a tough start to the [last] climb, everyone was on the limit but Wout [Poels] and Egan [Bernal] did a tremendous job," he explained.

Froome's struggles, however, gave him the confidence to power on.

"Froomey said with 5 or 4 kilometres to go that he was not feeling super. It gave me confidence, because if Froomey is suffering then everyone is suffering and I was feeling good," the 32-year-old Welshman said.

With that in mind, Thomas even pushed for a four-second time bonus allocated to the rider taking third place in the stage.

"In the last 200 metres I just thought I would go for the bonus seconds and get a little time gap as well," he said.

"It's the first time I've ridden for GC (general classification). I'm feeling good but I'm not going to get carried away. No complacency."

Despite feeling so good on the daunting 16km climb at an average gradient of 8.3 per cent, Thomas was not tempted to attack earlier, aware of how he could be affected by altitude — 2,215 metres at the summit.

"I didn't want to risk anything. At that altitude, as soon as you dig deep, it bites you in the ass," he said.

AP/ABC