Leo Cullen’s side were in outstanding form with seven different try-scorers crossing to ensure a 43-16 triumph and a five-point gap between themselves and Saints in Pool 1.
Elsewhere, Munster ousted reigning European champions Saracens 10-3 in a feisty encounter at Thomond Park to also stay unbeaten.
And Glasgow Warriors and Ulster Rugby also secured crucial wins, with the latter remaining top of Pool 3 at the halfway mark.
Leinster overpower Saints
The Guinness PRO14 champions travelled to Franklin’s Garden with both sides unbeaten after their opening two pool matches and there was little to separate the former European Cup winners at half-time.
James Lowe, who made life difficult all afternoon for Saints’ defenders, scored his seventh European try for Leinster with a breakaway score, while Rhys Ruddock and Andrew Porter also dotted down in the first half.
Wales international Dan Biggar kept Saints in the game with three penalties and a successful conversion of Ahsee Tuala’s early try to ensure the gap was only three points at the break but it was the visitors who gathered momentum quickly in the second period.
Four tries from Cian Healy, Ross Byrne, Luke McGrath and Ed Byrne left the current English Premiership leaders empty-handed with a return game in Dublin next weekend promising to be just as entertaining.
What a win today. 🙌
What support. 💙👊
— Leinster Rugby (@leinsterrugby) December 7, 2019
Munster take down the champs
Thomond Park was once again rocking for a fixture which has delivered more than its fair share of enthralling contests in Europe over the seasons.
Welcoming the defending champions Saracens, Munster started the more potent and were awarded for their ambition with Peter O’Mahony crossing for the game’s only try in the first half.
Penalties were traded between Saracens scrum-half Ben Spencer and Munster’s JJ Hanrahan as Munster led 10-3 at half-time.
The second period was scoreless as Johann van Graan’s men maintained their seven-point lead even in the face of Arno Botha’s late sending off to stay unbeaten in Pool 4, with the group still wide open.
Warriors roar past French opponents
Glasgow Warriors kept their hopes of a quarter-final place alive after securing a brilliant 27-24 win at La Rochelle.
The visitors were somewhat fortunate to go into the break only five points behind after La Rochelle scored two first-half tries through Dany Priso and Zeno Kieft.
But the Scottish club were determined to fight back in the second half with Niko Matawalu’s length-of-the-field breakaway try and substitute Kyle Steyn’s effort putting them ahead.
A tense finale ensued as Glasgow endeavoured to keep their French opponents out. A late lineout maul and scrum close to the Warriors line provided the hosts an opportunity but they were denied through outstanding determination from the Guinness PRO14 outfit.
✅ New contract
✅ First #ChampionsCup try
✅ Away win in France
— Glasgow Warriors (@GlasgowWarriors) December 7, 2019
Joy for Ulster, misery for Ospreys
Ulster snatched a stunning victory at the death courtesy, once again, of John Cooney’s sublime kicking to stay on top of Pool 3.
The back-and-forth encounter with Harlequins thrilled a sold-out Kingspan with the scores level at half-time.
Last season’s quarter-finalists, Ulster, upped their game in the second period, with try-scorer Stuart McCloskey particularly impressing, and a late penalty gave Cooney the opportunity to slot a kick between the stick to seal a one-point victory.
However, it was a disastrous day at the office for Ospreys after a humbling 40-19 defeat against Racing 92, as interim coaches Matt Sherratt and Carl Hogg look to find positives in an otherwise disappointing start to the season.
The Welsh side were not helped by an early red card to Dan Evans, who was sent off after just 37 seconds, leaving the hosts to futiley battle a star-studded Racing 92 for the majority of the game a man down.
Elsewhere, it was a lean day in Lyon for Benetton who left the south of France without working the scorers in their 28-0 defeat in Pool 1.
The loss leaves the Italian side all but out of the Champions Cup after three games as they sit rock bottom of the pool.
Challenge Cup action
Elsewhere, Zebre secured their first European win of the campaign in the Challenge Cup with a dramatic bonus-point victory over Brive in Pool 4.
Tries from Paula Balekana, Carlo Canna, Giovanni Licata and Jamie Elliot gave the Italian side a 27-24 home success.
Dragons provided Worcester Warriors a tough challenge at Sixways but were unable to find a way past as they fell 34-28.
However, Cardiff Blues overwhelmed Pau 54-22 to keep tabs on Leicester Tigers in Pool 5, while Scarlets came away from Bayonne with a 19-11 road success to keep them in touch with Toulon in Pool 2.