London: Barcelona conceded a goal after 24 seconds and another in stoppage time as its Champions League title defense opened with a 2-2 draw against AC Milan on Tuesday, while Arsenal was also left stunned by Borussia Dortmund's late equalizer.

Alexandre Pato opened the scoring for Milan and, after goals from Pedro Rodriguez and David Villa looked to have put Barcelona in control, Thiago Silva's header secured a point in Group H.

Dortmund leveled in the 88th minute, with Ivan Perisic's volley canceling out Robin van Persie's first-half goal for Arsenal in the 1-1 draw in Group F.

Chelsea manager Andre Villas-Boas oversaw a 2-0 victory over Bayer Leverkusen in Group E in his first Champions League match, while his former club Porto recovered to beat nine-man Shakhtar Donetsk 2-1 in Group G.

While Porto won the Europa League in May, Barcelona lifted the European Cup for the second time in three years.

The Wembley Stadium triumph cemented Barcelona's status as world football's superpower and Pep Guardiola's side is favorite to become European champions for the fifth time in May.

But the script went against them at the Nou Camp on Tuesday against a Milan side missing Zlatan Ibrahimovic and Robinho to injury.

Just 24 seconds had elapsed when Pato coolly slid the ball under goalkeeper Victor Valdes after dashing through Barcelona's half.

A determined Lionel Messi led the Spanish champions' response as the Argentina forward created Pedro Rodriguez's 36th-minute equalizer before David Villa curled a long-range free kick into the net five minutes into the second half.

"We could have played better after Barcelona took the lead," AC Milan coach Massimiliano Allegri said. "But I'm happy because the team did not collapse until we found the late equalizer."

That came deep into stoppage time when Thiago leapt high to meet Clarence Seedorf's corner to secure a point for Milan.

"We did all we were supposed to do but we are still left with this bitter taste," Barcelona coach Guardiola said.

All four teams in Group H are on one point after BATE Borisov and Viktoria Plzen drew 1-1.

In Germany, Arsenal went in front just before halftime when Van Persie won possession from Sebastian Kehl and played a one-two with Theo Walcott before beating Dortmund goalkeeper Roman Weidenfeller with his shot.

But after Marcel Schmelzer's free kick was only half-cleared by Kieran Gibbs toward the end, Ivan Perisic volleyed in a late equalizer.

The draw enabled Marseille to go top of the group after Lucho Gonzalez secured a 1-0 victory over Olympiakos.

The Argentine exploited a lapse in the Greek defense, meeting Jeremy Morel's cross before beating former River Plate teammate Franco Costanzo in the Olympiakos goal.

At Stamford Bridge, Fernando Torres continued his goal drought for Chelsea this season but set up both goals.

David Luiz ran onto Torres' layoff before bending a sublime strike into the bottom corner of the net midway through the second half.

And after Torres broke down the left flank, Juan Mata tapped home the second with almost the last kick of the game.

Chelsea is top of Group E after Racing Genk draw 0-0 with Valencia in the other match.

There were two victories in Group G - both with 2-1 scorelines.

And at Porto's Stadium of the Dragon, all the goals came from Brazilians.

Porto goalkeeper Helton gifted Shakhtar the lead after 12 minutes, failing to hold Willian's weak shot from outside the area and spilling the ball for Luiz Adriano to tap in.

But striker Hulk equalized for Porto directly from a free kick just before the half hour and Kleber latched onto James Rodriguez's cross to poke in the host's winner six minutes after half time.

Shakhtar lost left back Yaroslav Rakitskiy just before half time for a studs-up challenge on Joao Moutinho and center back Dmytro Chygrynskiy was shown his second yellow card in the 80th.

There was a first-ever Champions League win for APOEL Nicosia after second-half goals from Gustavo Manduca and Ailton secured a 2-1 win over Zenit St. Petersburg.

(Agencies)