Ronaldo grabbed two goals as Real Madrid came from behind to secure a 4-2 win at home to Marseille at the Bernabeu.
The French club went ahead through Didier Drogba but it lasted barely four minutes as Roberto Carlos equalised from a David Beckham cross.
Ronaldo tapped in from close range to put them ahead after great skill from an in-form Zinedine Zidane and the Brazilian added a second 11 minutes after the break.
Roberto Carlos was brought down in the box for Luis Figo to score a twice-taken penalty to put them 4-1 ahead, before Marseille defender Daniel van Buyten scored an 83rd-minute consolation.
Reigning champions AC Milan began the defence of their European crown with a hard-fought 1-0 victory over Ajax at the San Siro.
Filippo Inzaghi carried on his recent fine run of scoring, finding the net after Ajax keeper Bogdan Lobont had failed to hold a powerful shot from strike partner Andriy Shevchenko.
Despite Milan's domination they had Brazilian goalkeeper Dida to thank for denying Rafael van der Vaart an equaliser in stoppage time with a breathtaking one-handed save.
Milan's victory was soured by the late sending-off of midfielder Gennaro Gattuso, who was red-carded for lashing out at Zlatan Ibrahimovic.
In the other Group H game Club Bruges managed a 1-1 draw with Celta Vigo , despite finishing the match with just nine men.
Andres Mendoza was shown the red card after 44 minutes for retaliating against Jose Ignacio and Sandy Martens was sent off a minute from the end for two bookable offences.
Juanfran gave Champions League debutants Celta Vigo the lead after 48 minutes, before Bengt Saeternes equalised for Bruges six minutes from time.
In Group G goals from Jaap Stam and Stefano Fiore earned Lazio a 2-0 win over Turkish champions Besiktas .
For Besiktas it was a case of deja vu as the Romans that proved their conquerors in the Uefa Cup quarter-finals last season.
Stam scored with a header shortly before half-time for his first goal in the Champions League and Fiore ensured the three points with a goal 13 minutes from time.
Partizan Belgrade and FC Porto played out a 1-1 draw in Group F.
Partizan, the first Serbian side to qualify for the group stage of the competition, fell behind when midfielder Francisco Costinha headed in from close range after 21 minutes.
The home side looked livelier after the break and when veteran Ljubinko Drulovic's free-kick fell to striker Andrija Delibasic in the 54th minute he made no mistake.
It was a good night for the three British clubs, who all got off to a winning start.
Manchester United ran out 5-0 winners over Greek side Panathinaikos at Old Trafford.
Mikael Silvestre got the opener after 13 minutes and goals from Quinton Fortune, Ole Gunnar Solskjaer, Nicky Butt and Eric Djemba-Djemba completed the rout.
Rangers came from behind to record a fine win at home to VfB Stuttgart , with two goals in five minutes from Christian Nerlinger and Peter Lovenkrands.
Chelsea left it late but came away from Sparta Prague with what could be a vital three points thanks to a rare goal from defender William Gallas.
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Last season's beaten finalists Juventus opened their Champions League campaign with a scrappy 2-1 win over Galatasaray. Alessandro del Piero put the home side ahead after just five minutes, with Hakan Sukur levelling for the Turks 15 minutes later. But veteran defender Ciro Ferrara scored the winner from close-range after 73 minutes.
In the other Group D match, Real Sociedad edged past Olympiakos courtesy of a late Darko Kovacevic penalty.
The other Spanish side in action on Wednesday, Deportivo La Coruna , were also taking on Greek opposition as they travelled to Athens.
And they too looked to heading for a 1-0 win when Walter Pandiani gave them a 12th-minute lead.
But AEK hit back to grab a point through Vassilis Tsartas five minutes from the end of normal time.
Also in Group C, Monaco survived a late fightback from PSV Eindhoven to win 2-1 in Holland.
On-loan Real Madrid striker Fernando Morientes headed the visitors into the lead after half an hour and they doubled their advantage through Edouard Cisse's precise finish after 55 minutes.
Wilfred Bouma pulled one back for PSV with a stunning free-kick, but Monaco held on to take the points.
In Group B, Diogo Rincon came off the bench to score twice in the last 10 minutes to give Ukrainian side Dynamo Kiev a 2-0 win over Russian rivals Lokomotiv Moscow .
The Brazilian headed home Valentin Belkevich's cross to put the hosts ahead after 83 minutes and he doubled Kiev's advantage when he latched onto another ball from Belkevich and drilled a low shot into the corner.
Inter Milan ran riot in the other game in the group, thrashing Premiership leaders Arsenal 3-0 at Highbury.
All Inter's goals came in the first half with Julio Ricardo Cruz, Andy Van Der Meyde and Obafemi Martins finding the target. Thierry Henry missed a penalty for the Londoners.
A 26th-minute penalty from Juninho Pernambucano gave French side Lyon a 1-0 win over Belgian rivals Anderlecht in Group A.
And Bayern Munich fought back from trailing to a 57th-minute Alan Thompson goal to beat Celtic 2-1, with Roy Makaay getting both the goals for the German giants.
Story from BBC SPORT:
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