The Top 14 side - winners of the Heineken Cup in 2013 and 2014 - lost 25-21 at Welford Road in a match that was marred by Martin Castrogiovanni's scatching verbal attack on his former club and director of rugby Richard Cockerill.
The Italian, who has since been charged by European Professional Club Rugby, was not involved on Saturday, but his team-mates were in control throughout after piling on the points in the early stages.
Lock Ali Williams powered over after 10 minutes and, along with 11 points from the boot of Leigh Halfpenny, he helped put Toulon into a commanding 16-0 lead.
Blaine Scully reduced the deficit by five before the half-time whistle, but Mathieu Bastareaud's converted try 14 minutes into the second half left Leicester with an uphill struggle to claw their way back.
Owen Williams' penalty just before the hour proved to be the final score of the match in an otherwise disappointing outing for the Welshman, as Toulon went five points clear at the top of Pool Three.
"It was as simple as the fact that we were inaccurate and you have to be accurate on the big stage," said Cockerill. "Owen Williams had a tough day at the office but he is a young guy learning his trade. He will be better for it.
"Physically we were beaten today by a big strong team.
"Second place is still a realistic position for us. Whether we can get enough points out of our two games though, we will have to see."
Northampton Saints opened up a one-point lead at the top of Pool Five with a thumping 67-0 win over Benetton Treviso which saw the English side score 11 tries.
Ken Pisi and substitute Samu Manoa scored three apiece, with Saints scoring 43 of their points after the interval.
The result sees them go clear of Racing Metro, who held off a late fightback from Ospreys to emerge as 18-14 victors on Saturday.
There was another late show in Dublin, where Harlequins battled back from 11-0 down at the break to lead Leinster 13-11 heading into the final 20 minutes.
However, Ian Madigan found the posts to deny the visitors in a fiery encounter that ended 14-13.
Sale Sharks' hopes of qualification are over after they lost 28-15 to Saracens, who go top of Pool One for the time being, with Clermont Auvergne and Munster due to meet on Sunday.
In Pool Four, Toulouse stretched their lead to six points with a 12-9 win at Glasgow Warriors.