Former champions Ulster got off to a winning start in the Champions Cup with a 17-16 win at Bath on Saturday, while Clermont marked their return with a seven-try demolition of Harlequins.
Four-time champions Leinster were too strong for Benetton and there were opening day wins for Exeter at La Rochelle and Glasgow Warriors at home to Sale.
Ulster scrum-half John Cooney scored the opening try after 10 minutes of the Pool 3 encounter and converted for a 7-0 lead although two penalties from Freddie Burns meant the teams turned around at half-time with just a point between them.
Right-wing Rob Lyttle showed his pace just before the hour to chase down a kick through from Will Addison and touch down for Ulster's second try.
Minutes later Bath, the 1998 champions, hit back with replacement wing Gabriel Hamer-Webb scorching down the left for a try.
A Rhys Priestland penalty after 67 minutes put Bath ahead for the first time but four minutes later Cooney was on target once more to restore Ulster's one-point lead.
The home side almost stole it at the death. With the time three minutes into the red, number eight Zach Mercer passed outside to wing Semesa Rokoduguni who had the line at his mercy.
The ball never arrived, however, as Ireland wing Jacob Stockdale flung himself in the way to intercept the pass and tumble into touch, signalling the end of the match.
"Probably not the best, expansive rugby but boy did we show some heart and desire to hang in that fight," said Ulster fly-half Billy Burns.
"There was a lot of times we were under pressure but we stuck in there and it was great to get the result."
- Crushing wins -
Fijian flanker Peceli Yato and France wing Alivereti Raka scored two tries apiece with Isaia Toeava, George Moala and Mike Tajder also crossing in a 53-21 rout of Harlequins.
Morgan Parra added 16 points with the boot as Clermont, who failed to qualify for last season's competition, made a terrific start to their Pool 3 campaign.
"It was a good performance tonight," said Clermont head coach Franck Azema. "There are always things to improve but... I've been waiting for a match like that for a while."
"Peceli Yato's display was of great interest. He'd already made a good return last week but tonight he was solid and we know about his finishing ability."
Returning centre Garry Ringrose helped himself to a hat-trick as Leinster, beaten in last year's final by Saracens, dismantled Italian side Benetton 33-19 in their Pool 1 meeting in Dublin.
Ringrose touched down twice inside the first 14 minutes, sandwiching a Dean Budd effort, and Ronan Kelleher marked his European debut with a try on the half-hour mark as last season's runners-up opened up a 19-7 interval lead.
Leinster captain Jonathan Sexton converted his own bonus-point try but Hame Faiva hit back to keep the pressure on.
Man-of-the-match Ringrose cut inside a tired 68th-minute tackle to notch his first career hat-trick, before Luca Sperandio claimed a late consolation score.
Munster eased to a 32-13 victory at Welsh side Ospreys in Pool 4 courtesy of tries from Jeremy Loughman, Keith Earls, Andrew Conway and James Cronin.
English side Exeter were also in rampant mood as they thumped La Rochelle 31-12 in France in Pool 2.
Dave Dennis and wing Tom O'Flaherty touched down in the first half to give Exeter a 14-0 lead at the break.
Henry Slade and Sam Simmonds touched down in the second half with La Rochelle crossing the paint through Geoffrey Doumayrou and Kini Murimurivalu.
In the other game in Pool 2, DTH Van Der Merwe's try gave Glasgow Warriors the edge as they beat Sale 13-7.
Dave Rennie's team dominated the first half but should have taken more than a solitary try and eight points from the boot of Adam Hastings.
Sale, who were missing World Cup winning scrum-half Faf de Klerk and England's Tom Curry and Mark Wilson, struggled for rhythm although replacement prop Coenie Oosthuizen did crash over for a try in the second half.
Peceli Yato (C) scored two tries as Clermont romped to an opening Champions Cup win over Harlequins