Borussia Dortmund and Chelsea recovered from poor starts in the Champions League by winning their first group matches on Tuesday, while Barcelona beat 10-man Celtic 1-0 to avoid a second consecutive loss at Celtic Park.
Robert Lewandowski scored a brace as Dortmund, last-season's runner-up, defeated Marseille 3-0 in Group F. Arsenal leads the group with six points after a 2-0 win over Napoli.
Chelsea kick-started its campaign with a 4-0 rout of Steaua Bucharest in a Group E dominated by Schalke. Julian Draxler made sure the German side maintained its 100 percent record this season with a superb half-volley that earned his side a 1-0 win at Basel, where activists protesting against Champions League sponsor Gazprom abseiled from the stadium roof and forced the match to be briefly suspended.
Missing the injured Lionel Messi, Barcelona earned a workmanlike second win in Group H, courtesy of Cesc Fabregas' strike at the scene of last season's shock defeat.
In the group's other match, Mario Balotelli converted a penalty in the fourth minute of injury time to give AC Milan a 1-1 draw against Ajax.
Ramires scored twice to help Chelsea get its Champions League campaign back on track following a surprise 2-1 home defeat by Basel to open Group E.
"We played good football with intelligence and maturity," Chelsea manager Jose Mourinho said. "We needed to clean what we did at home (against Basel) and we did it with a good result and performance. Now we are not in a dramatic situation, we are in a close situation."
The Blues dominated throughout and opened the scoring in the 20th minute through Ramires. They doubled their lead on the stroke of halftime when Daniel Georgievski scored an own goal before Ramires and Frank Lampard sealed Chelsea's victory after the break.
Atletico Madrid moved on to six points as second-half goals from Diego Godin and Arda Turan helped the Spanish side to a 2-1 win at Porto in Group G.
Dortmund was in a different class to Marseille and took the lead when Lewandowski ended a counter-attack with a clinical finish in the 19th minute. As Dortmund coach Juergen Klopp watched from the stands after being banned for one match, the tall striker then converted a penalty in 80th. Marco Reus was also on the score sheet as Marseille slipped to its second consecutive defeat.
"We played a very good game, but we have to play the next one just as well," Lewandowski said.
At the Emirates, Mesut Ozil again proved to be an excellent signing for Arsene Wenger's team as the Gunners took command of the group, three points clear at the top ahead of Dortmund's visit on Oct. 22.
Ozil's first goal since becoming Arsenal's most expensive player and another strike from Olivier Giroud within the opening 15 minutes were ideal presents for Wenger, who celebrated 17 years in charge at the London club.
Ozil sidefooted into the net in the eighth minute after Giroud chested down the ball and flicked it onto Aaron Ramsey, who delivered the cross that the playmaker met.
"It is everything that you dream to see when you sit in the stands watching football," Wenger said. "It's always on your mind a little bit (scoring your first goal) so he got rid of it."
Ozil then assisted Giroud, pulling back the ball for the former Montpellier forward to score in the 15th minute.
In a game short of chances, Barcelona had to wait until the 75th minute to avoid another frustrating night in Glasgow, having slumped to a shock defeat to Celtic in the group stage last season.
The Spanish giants capitalized on the sending-off of Celtic captain Scott Brown, who was given a straight red card in the 59th minute for kicking out at Neymar.
Celtic's overworked defense failed to hold out for the final half hour as Fabregas headed a cross by substitute Alexis Sanchez into the corner, earning Barcelona a second straight victory in Group H after beating Ajax 4-0.
In Porto, Turan scored the winner four minutes from time as Atletico Madrid rallied to secure a 2-1 win.
The result sent the Spanish club to the top of Group G, three points clear of Porto. Zenit St. Petersburg and Austria Vienna drew 0-0 in the other group game and have one point each.
Atletico is unbeaten this season and defeated Real Madrid in the Spanish league last weekend. But it was tested by two-time European champion Porto's strong first-half performance, with Jackson Martinez scoring from a diving header in the 16th before Godin and Turan struck after the break.