Dimitar Berbatov's first two goals for Manchester United helped secure a 3-0 win over AaB Aalborg in Champions League Group E on Tuesday, while Juventus had to fight back for a 2-2 draw at BATE Borisov in Group H.
The Bulgarian striker pounced twice in the second half after Wayne Rooney had put the holders ahead in the 22nd minute to give United their first win of the campaign.
Man Utd bt AcB 3-0
Arsenal bt FC Porto 4-0
BATE Borisov, Juventus 2-2
Bayern, Lyon 1-1
Fenerbahce, Dynamov Kiev 0-0
Fiorentina, S Bucharest 0-0
Villareal bt Celtic 1-0
Real Madrid bt Zenit 2-1
Villarreal are level at the top of the group with United on four points after a 1-0 win at home to Celtic with a second half goal from midfielder Marcos Senna. Celtic and Aalborg have a point apiece.
Juventus had to come from two goals down in Minsk with BATE holding on for their first-ever Champions League point. Vincenzo Iaquinta scored both Juve goals after the Belarus side went two up in 23 minutes through Sergei Kryvets and Igor Stasevich.
It was the second time a Serie A side suffered a shock result this season in European soccer's top competition after Romanian debutants CFR Cluj defeated AS Roma in their opening match a fortnight ago. Cluj face Chelsea on Wednesday.
"They surprised us. We couldn't hold them in any area of the field. It was a nightmare first half hour," Juve coach Claudio Ranieri told Sky television.
"We did well not to give up. We have to compliment these lads from Belarus. They ran like there were 22 of them, not 11."
In the other Group H match, Real Madrid kept their 100 percent record intact with a 2-1 win at UEFA Cup holders Zenit St Petersburg, Ruud van Nistelrooy netting the 31st-minute winner.
Real, who went ahead with a fourth-minute own goal by Tomas Hubocan before Portugal's Danny equalised, lead the group on six points after two games. Juventus are second with four points, BATE have one and Zenit none.
Arsenal crushed Porto 4-0 in Group G with a double strike from Robin Van Persie and goals in each half from Emmanuel Adebayor, his second a penalty. The other group match between Fenerbahce and Dynamo Kiev ended scoreless.
"I expect us to win the group from here," Arsenal manager Arsene Wenger told reporters.
"It was what I expected, we could have scored more but we responded in a convincing way," he said referring to Saturday's shock 2-1 home defeat by Hull City in the Premier League.
Arsenal have four points from two games, one point more than Porto. Dynamo have two points and Fenerbahce one.
Bayern Munich top Group F despite being held 1-1 at home by Olympique Lyon. The French side took the lead when Bayern's Martin Demichelis headed into his own net in the 25th minute, but the German champions claimed a crucial point when Ze Roberto headed home after the break.
Bayern have four points with Lyon and Fiorentina, held 0-0 at home by Steaua Bucharest, two points behind. The Romanians have one point.