Celtic stunned Ajax in Glasgow with a 2-1 victory that leaves them well placed in Group H, thanks to Milan and Barca's draw.
James Forrest grabbed their first from the penalty spot after Stefano Denswil had fouled Anthony Stokes before Baram Kayal's shot from range took a deflection to find the bottom corner.
Lasse Schone registered a consolation goal for Ajax deep into stoppage time with a fierce strike from range but Celtic saw out the game to seal another famous home win.
In the biggest game of the evening, AC Milan and Barcelona shared the spoils in a thrilling match at the San Siro.
Milan edged ahead inside ten minutes when Robinho capitalised on some poor Barca defending and swapped passes with Kaka before finishing from close range.
But Lionel Messi drew the visitors level soon after, cutting inside from the right flank before sliding a low shot into the bottom corner.
Both sides had chances to pinch a winner - Andres Iniesta forcing a desperate save from Marco Amelia, while Robinho miscontrolled inside Barca's penalty area with only Victor Valdes to beat.
Atletico Madrid breezed past Austria Vienna with an emphatic 3-0 victory at the Franz Horr Stadium.
Atletico cruised into a two-goal lead inside 20 minutes in Vienna, Koke's pass releasing Filipe Luis into the area and he squared for Raul Garcia to tap home.
Twelve minutes later Diego Costa doubled Atletico's advantage, charging from inside his own half before finding the net and the Brazilian put the gloss on the result by finishing neatly from Emiliano Insua's cross.
In Arsenal's group, Napoli saw out a nervy final few moments in France as they secured a 2-1 away win at Marseille.
Jose Maria Callejon put the Italians ahead shortly before half time, latching on to a Gonzalo Higuain pass before cutting inside and finding the net.
Then substitute Duvan Zapata scored with his second touch, rifling a rocket of a shot into the top corner from 25 yards out, to leave Napoli comfortable.
Marseille gave themselves a sniff of a draw after Andre Ayew ran on to a pass from brother Jordan to loop a shot over Pepe Reina but Rafa Benitez's side held on for the win.
Porto sunk to a 1-0 home defeat at the hands of Zenit St Petersburg thanks to Alexander Kerzhakov's late goal.
Hector Rubio was sent off inside six minutes after picking up two yellow cards in the space of 20 seconds - one for a foul and the second for encroaching from a free kick.
Porto held on until six minutes from time when Kerzhakov found some space inside the area and nodded home Hulk's cross.
A late equaliser from Leandro Tatu earned a point for Steaua Bucuresti as they drew 1-1 with FC Basel in Romania - the other two teams in Group E alongside Chelsea.
Marcelo Diaz's precise curling shot put the Swiss side ahead just after half time but Tatu stole in to finish from a low ball into the area as Steaua grabbed their first point of the competition.
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