Cardiff score a bonus-point win, Sale draw with Toulouse in thriller, while Agen and Bordeaux register big victories in Challenge Cup openers.
Cardiff got their Pool 2 campaign up and running with a bonus-point victory over Top 14 leaders Lyon with the Welsh outfit emerging 29-19 victors at Cardiff Arms Park.
After waiting 22 minutes to break the deadlock, the Blues fashioned converted tries from George Earle and Tomos Williams to earn a 14-0 half-time lead.
Macauley Cook's try stretched Cardiff's lead further before Lyon took advantage of Kristian Dacey's 58th-minute yellow card to open their account through Virgile Bruni's score.
However, Cardiff nipped any Lyon fightback in the bud when Rey Lee-Lo crossed for their fourth try, putting the hosts 26-7 ahead and earning them a bonus point.
Lyon grabbed two tries towards the end through Pierre-Louis Barassi and Jone Tuva - the second when down to 13 men - but it was too little too late.
At the AJ Bell Stadium, Sale began their European Challenge Cup campaign with a thrilling 20-20 draw with Toulouse.
The Sharks, having lost four of their opening six Aviva Premiership matches, looked set for another defeat when tries from Semi Kunatani and Sebastien Bezy - both converted by the latter - put Toulouse 14-3 up heading into half-time.
However, Sale hit back and built a 17-14 lead after prolific winger Denny Solomona and Bryn Evans crossed either side of the interval, with Faf de Klerk adding the extras.
Then it became a battle of the boot, with Bezy and De Klerk exchanging penalties to leave Sale holding a narrow 20-17 lead going into the closing stages.
But Bezy had the final say after Sale had been penalised for offside at a ruck the France fly-half nailing his 74th-minute kick to earn four-time European champions Toulouse a draw.
It was a better day for two other French clubs, with Bordeaux and Agen both recording big opening wins.
Bordeaux ran in eight tries as they thrashed Enisei-STM 57-17 in Pool One in the afternoon kick-off, while Agen ran out 45-10 victors over Zebre in Pool Three with fly-half Mathieu Lamoulie finishing with 18 points from two tries, a conversion and two penalties.