Lionel Messi hit a hat-trick as Barcelona began their Champions League campaign with a win over Ajax but Chelsea suffered a shock home defeat. (All AFP images)
AC Milan left it late before avoiding an embarrassing slip-up at the San Siro as Emilio Izaguirre's own goal broke Celtic's hearts, with Sulley Muntari adding a second to secure the Rossoneri a 2-0 success.
Miranda put them in front before powerful Brazil forward Hulk equalised. But goals from Arda Turan and Leo sent Atletico to the top of Group G, ahead of Porto, who won 1-0 at Austria Vienna thanks to Lucho Gonzalez's goal.
Jose Mourinho's Chelsea were stunned in the shock result of the night as FC Basel left Stamford Bridge with a 2-1 victory in Group E.
Brazilian midfielder Oscar had given Chelsea the lead just before half-time at Stamford Bridge on Wednesday. But manager Jose Mourinho was keen not to criticise lacklustre Willian, who was making his debut after a £30 million move from Anzhi Makhachkala.
Last season's runners-up Borussia Dortmund had a disastrous start to their campaign as they went down 2-1 at Napoli in Group F.
Lionel Messi took his tally to 62 Champions League goals and became the first player to score more than four Champions League hat-tricks, whilst also netting his 24th such treble for Barca. Barcelona beat Ajax 4-0.
Gerard Pique also scored a goal when he headed Neymar's cross past the stranded Kenneth Vermeer before Messi rounded off the scoring 15 minutes from time.
Schalke 04 took command of the group with three goals in the final quarter to beat Steaua Bucharest 3-0.