Swiss minnows Young Boys stunned Premier League giant Manchester United in a shock 2-1 defeat, despite a goal from Cristiano Ronaldo, in their Champions League opener Tuesday.

Current champion Chelsea kicked off its title defense by beating Zenit, while Robert Lewandowski struck twice for Bayern Munich as the German heavyweight inflicted a 3-0 defeat on Barcelona at Camp Nou in its first European game since the departure of Lionel Messi.

Ronaldo, the leading scorer in Champions League history, bagged his 135th goal in the competition after 13 minutes in Bern, but Swiss champions Young Boys hit back after the sending-off of Aaron Wan-Bissaka.

The United defender saw red for a dangerous tackle on Christopher Martins before halftime. Nicolas Moumi Ngamaleu equalized in the second half before Jordan Siebatcheu snatched the winner with the final kick.

"It's the first game in the group. We've got many games to bounce back and we must do," said United captain Harry Maguire.

"We'll try and pick up three points in our next game and build momentum from there."

United hosts Villarreal in two weeks. The Spaniards, who beat Ole Gunnar Solskjaer's men in last season's Europa League final, drew 2-2 at home to Atalanta in the other Group F match.

Remo Freuler gave Atalanta the lead but Villarreal responded with goals from Manu Trigueros and Arnaut Danjuma. Robin Gosens popped up with a late equalizer.

Lukaku guides the Blues

Romelu Lukaku broke the deadlock for Chelsea after 69 minutes at Stamford Bridge to earn the holder a 1-0 victory over Zenit, whose Krestovsky Stadium in St. Petersburg is the venue for next May's final.

Lukaku headed in Cesar Azpilicueta's cross to give Thomas Tuchel's team a winning start in Group H.

"The guys who are there to score regularly for their teams are so important because one goal changes the whole momentum of the match," Tuchel said of Lukaku.

Juventus top the section after the opening round following its 3-0 win away to Malmo of Sweden.

Alex Sandro headed Juve ahead and Paulo Dybala converted a penalty before Alvaro Morata notched a third for a side that has picked up just one point from three games in Serie A.

Bayern brushes Barca aside

Bayern stretched its record unbeaten away run in the Champions League to 19 games by sweeping aside Barca in the first meeting between the clubs since the Germans triumphed 8-2 in a one-off quarterfinal last year.

Thomas Mueller put Bayern on top on 34 minutes and Lewandowski netted twice in the second half to send Julian Nagelsmann's side top of Group E.

"We came to win this game and we knew that we had to show respect to Barcelona," said Lewandowski.

"When you play at the Camp Nou against them they are always dangerous. But we had this game under control and we showed we were here for the three points."

Dynamo Kyiv had to settle for a 0-0 draw at home to Benfica after Ukraine international Mykola Shaparenko's injury-time effort was ruled out for offside.

Sevilla rode its luck in an eventful 1-1 draw against Salzburg.

The Austrians missed two of three first-half penalties, with Karim Adeyemi firing wide from the spot and Luka Sucic hitting the post having earlier converted Salzburg's second penalty.

Ivan Rakitic leveled for Sevilla with another spot-kick before the interval. Youssef En-Nesyri was sent off early in the second period for the home side.

"It was a very strange game. When you have three penalties against you, a red card, I think you have to be happy with a draw," said Rakitic.

"I think it's a record, three penalties (conceded) in 36 minutes, I don't think it's ever happened to me."

French champion Lille and Bundesliga leader Wolfsburg drew 0-0 to leave all Group G teams level on one point.