After all the hype about Pep Guardiola returning to face his former club, Lionel Messi made sure it was no contest.

Messi scored a hat trick to deal a humbling defeat to his former mentor on Wednesday, leading Barcelona to an emphatic 4-0 victory over Manchester City in the Champions League.

Messi sped through City's defense and around former Barcelona goalkeeper Claudio Bravo before giving the hosts the 17th-minute lead in a Group C match that both teams finished with 10 men.

It only got worse for Bravo, who was sent off in the second half for using his hands to block a shot outside the area after his poor pass gifted the ball to Luis Suarez.

Messi struck again in the 61st and 69th to take his total to 89 goals in Europe's top club competition. The Argentina forward is second only to Cristiano Ronaldo's 98.

Barcelona's substitute defender Jeremy Mathieu was sent off in the 73rd minute for a second yellow card.

Neymar rounded off the win from a pass by Messi, moments after the Brazil striker had missed a penalty that Messi had earned.

Guardiola lost for the second time upon returning to Camp Nou where he established himself as one of the world's top managers by leading Barcelona to 14 titles from 2008-12. The former Barcelona midfielder coached Bayern Munich when the German side lost 3-0 at the Catalan side in the Champions League semifinals two years ago, when Messi scored twice.

Barcelona leads Group C with a perfect nine points, City has four and Borussia Moenchengladbach three after its 2-0 win at last-place Celtic, which has one point.

Here's a look at what else happened in the Champions League on Wednesday:

GROUP A

Mesut Ozil fired a second-half hat trick as Arsenal took a big step toward the knockout stage with a 6-0 rout of Bulgarian champion Ludogorets.

The north London side, which is now unbeaten since the opening game of the season two months ago, tops the group with seven points.

Alexis Sanchez, Theo Walcott and Alex Oxlade-Chamberlain had put Arsenal up 3-0 before Ozil took over.

Paris Saint-Germain also has seven points after its 3-0 home win over Basel, which is level on one point with Ludogorets.

Angel Di Maria ended his goal drought at just the right moment and striker Edinson Cavani continued his scoring streak as PSG overcame a poor start at the Parc des Princes.

Having gone 17 games without a goal for club and country, Di Maria pounced five minutes before halftime and Brazilian winger Lucas slotted in his seventh goal of the season in the 62nd before Cavani added a late penalty.

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GROUP B

Vincent Aboubakar scored twice as Besiktas beat Napoli 3-2 to deny the Italian club a chance of advancing from the group stage in record time.

Aboubakar, a Cameroon international on loan from Porto, scored the winner four minutes from time after having also restored Besiktas' lead late in the first half.

It was the Turkish side's first win in the competition since 2009.

If Napoli had won, it could have become the first team in Champions League history to advance from its group after just three matches.

Now, Napoli has just a one-point lead over Besiktas, with Benfica two points back after its 2-0 win at Dynamo Kiev.

After Adriano's opener for Besiktas, Dries Mertens and Manolo Gabbiadini scored temporary equalizers for Napoli.

In Kiev, Eduardo Salvio converted a penalty and Franco Servi also scored in Benfica's first win of the campaign.

GROUP C

In Glasgow, Borussia Moenchengladbach capitalized on two defensive errors by Kolo Toure to beat Celtic 2-0, earning the German team its first points.

Toure's first error came when he tried to shepherd the ball over the byline for a goal kick in the 57th. Andre Hahn wrapped his foot around the defender and hooked the ball back into the area, where Lars Stindl drove a shot through goalkeeper Craig Gordon's legs from an acute angle.

Hahn doubled the lead in the 77th after Toure's pass near the halfway line was intercepted. Hahn sprinted through the middle and toe-poked a finish high into the net from just inside the area, just as Toure got back to attempt a lunging tackle.

GROUP D

Bayern Munich dispelled any doubts about its commitment by ending a three-match winless run with a comfortable 4-1 victory over PSV Eindhoven.

Thomas Mueller opened the scoring in the 13th, Joshua Kimmich doubled the lead in the 21st, before Luciano Narsingh pulled one back for the visitors.

Robert Lewandowski scored in the 59th, just as PSV seemed to be making a comeback and Arjen Robben provided the insurance in the 84th.

Bayern has six points, three behind group leader Atletico Madrid, which won 1-0 in Rostov. PSV and FC Rostov have one point each.

In Rostov, Yannick Ferreira Carrasco fired in the winner shortly after the hour mark for Atletico, last season's runner-up.

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