Sagan captures Tour's first stage, Cancellara keeps yellow jersey
Defending champion Cadel Evans and fellow title contender Bradley Wiggins of Britain trailed close behind in the pack after the 123-mile loop from Liege to suburban Seraing featuring five low-grade climbs.
Sagan, at age 21 one of cycling's most promising riders, placed his hands on his shoulders as he collected his first Tour stage win in a three-man sprint ahead of Cancellara in second and Edvald Boasson Hagen of Norway in third.
Cancellara, who won the Tour's opening-day prologue Saturday, leads Wiggins in the overall standings by 7 seconds. France's Sylvain Chavanel is third with the same time. Evans is 17 seconds behind the Swiss leader.
The 198-rider pack split up during a final climb, when the stage turned into a three-man race. Sagan hugged the wheel of Cancellara, doing the hard work of leading into the wind, then whipped around him less than 150 yards before the finish to win in 4 hours, 58 minutes, 19 seconds.
"I am really, really happy," Sagan said. "I was the only one who could follow (Cancellara). I was tight behind him. I was just happy to stay on his wheel."
It was the Liquigas-Cannondale rider's 13th stage victory this season, after winning in races as diverse as the Tirreno-Adriatico, the Tour of Switzerland and the Tour of California - where he won five of the race's eight stages.
At least two crashes marred Sunday's stage amid escalating tensions within the pack near the finish, where roadside crowds drew in to get a glimpse of the whirring cyclists.
High-profile riders including Spain's Luis Leon Sanchez and Michael Rogers of Australia went down in one late spill, but got back up. Bad luck continued to plague Germany's Tony Martin, who went down in a crash early Sunday before recovering. The world time-trial champion popped a flat and lost in the prologue the day before.
At one point, with his BMC team leader Evans riding in his wake, Marcus Burghardt of Germany caused his bike to jump to avoid a plastic bottle in a downhill patch about 10 miles before the finish.
Wiggins wore the best sprinter's green jersey after placing second in Saturday's prologue - an honor granted to him because Cancellara cannot wear both the green and the yellow jersey.