Lionel Messi scored twice to lead Barcelona to a 3-0 win over Juventus in Tuesday's Champions League opener, earning a measure of revenge for last year's ouster.
Four months after Juventus eliminated them in the quarterfinals, Barcelona began the group stage with a victory at the Camp Nou as Messi scored just before the break and helped set up a second before adding Barca's third himself.
Ousmane Dembele, making his first Barcelona start since his €150 million move from Dortmund this summer, nearly broke through after intercepting a foolhardy pass from Mattia De Sciglio in the box, but he could only fire wide.
De Sciglio had to come off with a knock soon afterwards, and Messi provided the opener just before the break by rolling a shot inside the far post after a one-two with Luis Suarez.
Messi nearly had a second goal soon after the break, but his shot from 20 yards shook the post, then came back off of Gianluigi Buffon before falling out of play.
The Argentine then played a large role in Barcelona's second as he ran the ball to the endline, and his cross across the face of goal was only cleared so far as the middle of the box, where Ivan Rakitic was on hand to lash home.
Messi then ran past two stationary Juventus defenders to enter the box and add his second goal in the 69th minute.
In the other game in Group D, Sporting Lisbon secured a 3-2 win away to Olympiakos.
It took less than two minutes for Seydou Doumbia to head home the opener, and Gelson Martins had the visitors 2-0 up inside 13 minutes following a long run on the counter-attack.
Bruno Fernandes got behind the defence two minutes before half-time to add a third as the Portuguese side went top of the group despite Felipe Pardo's late brace providing at least some consolation for the hosts.
Manchester United marked their return to Champions League football with a comfortable 3-0 home win over Swiss side Basel.
United made a fast start, with striker Romelu Lukaku having an early shot blocked and Ashley Young shooting wide soon afterwards.
But they were rocked after 19 minutes when midfielder Paul Pogba, stretching for a challenge, suffered what looked to be a thigh injury and was replaced by Marouane Fellaini.
Jose Mourinho's side were within a whisker of taking the lead when Henrikh Mkhitaryan's low strike cannoned back off a post, with the same player seeing his effort on the rebound blocked.
But they had the advantage when Young crossed and substitute Fellaini headed home to break the deadlock with 10 minutes remaining until the break, and the lead was doubled when Romelu Lukaku thumped home a header from a corner early in the second half.
Basel almost pulled a goal back when David De Gea was forced to make a brilliant save to deny Mohamed Elyounoussi after he had produced a sharp turn and shot, but with seven minutes remaining United made it three when Marcus Rashford finished off a Fellaini cross.
Also in Group A, CSKA Moscow came from a goal down to win 2-1 at Benfica.
The hosts could twice have taken the lead in a flurry of action just after the half-hour as Jonas struck an attempt narrowly off target before Alex Grimaldo came closer still when his effort crashed back off a post.
Benfica needed just five minutes of the second half to break the deadlock, when Haris Seferovic finished off a pass by Andrija Zivkovic, but CSKA were level just after the hour when Vitinho scored from the spot after a foul following a corner.
And with 20 minutes to play, the Russian side completed the turnaround when Timur Zhamaletdinov took advantage of another corner.
A blistering first half performance by Paris Saint-Germain saw their star forward line all on target as Scottish champions Celtic were thrashed 5-0 at Celtic Park in Group B.
PSG made the majority of the early running and had chances before, with 20 minutes gone, Neymar broke through in style when he clipped home a fine finish.
The Scottish champions were not lying down and Leigh Griffiths saw a free kick tipped away by Alphonse Areola, but they were two down when Kylian Mbappe picked up a loose ball to put PSG firmly in control after 34 minutes.
Any faint hopes of a Celtic revival were snuffed out five minutes before the break as Jozo Simunovic pulled back Cavani inside the area and the striker slotted home the penalty.
Brendan Rodgers' side conceded a late fourth when Mikael Lustig put the ball into his own net, and with five minutes remaining Cavani rubbed salt into Celtic wounds with the fifth.
Bundesliga champions Bayern Munich began their campaign with a comfortable 3-0 home win over Belgian champions Anderlecht.
Bayern dominated the early stages, with James Rodriguez failing to convert an excellent headed chance, but led after 12 minutes as Andrlecht were reduced to 10 men.
The incident happened when Sven Kums pulled back striker Robert Lewandowski inside the area, with the referee showing him a red card and awarding a spot kick that Lewandowski converted.
Bayern began the second half on the front foot and their pressure told after 65 minutes when Joshua Kimmich's low cross was converted by Thiago Alcantara.
Kimmich got the goal his performance had deserved when he rounded off the scoring in the 90th minute.
In Group C, new signing Davide Zappacosta scored a spectacular goal on his full Chelsea debut in a simple 6-0 win over Qarabag.
Pedro Rodriguez powered home the opener from 20 yards in the fifth minute, but it was Zappacosta's strike on the half-hour that will grab the headlines.
The defender received the ball from Thibaut Courtois not far from his own goal and turned up the speed, evading two defenders on the sideline before firing into the net from the side of the box as the Qarabag keeper was expecting a cross.
Cesar Azpilicueta's header made it 3-0 in the 55th minute, before substitute Tiemoue Bakayoko also scored his first goal for Chelsea as the ball fell to him in the box in the 71st minute.
Michy Batshuayi completed the rout by adding a brace with goals in the 76th and 82nd minutes, first scoring with a shot from outside the box before bundling a second over the line.
Chelsea enjoy an early two-point lead atop the group as rivals Roma and Atletico Madrid played out a goalless draw at the Stadio Olimpico.