Thiago Silva powered home a header in the second minute of stoppage time to steal a 2-2 draw for AC Milan at holders Barcelona in the heavyweight pair's Champions League Group H opener on Tuesday.

Barca had recovered from conceding the fifth quickest ever goal in Europe's elite club competition, scored by Alexandre Pato after 24 seconds, through strikes from Pedro and David Villa.

Once ahead they quickly settled into a rhythm, enjoying almost 70 percent of possession, but failed to kill off the seven-times champions and surrendered the lead for a second consecutive game following Saturday's 2-2 La Liga draw at Real Sociedad.

Their disappointment was compounded by the news that midfielder Andres Iniesta will be out for around four weeks after tearing a leg muscle shortly before halftime.

"It wasn't easy trying to penetrate the opponent's penalty area crowded with eight defenders," Barca coach Pep Guardiola told a news conference.

"We tried but at the end they levelled from a corner. What Milan did has a lot of merit. They made it as far as our goal two and a half times and they came away with a point."

Around 90,000 home fans packed into a sultry Nou Camp were stunned into near-silence seconds after the start when Brazil striker Pato left the home defence for dead and beat goalkeeper Victor Valdes with a low shot.

With captain Carles Puyol only declared fit this week after knee surgery and his partner in central defence Gerard Pique sidelined, Guardiola deployed midfielders Sergio Busquets and Javier Mascherano at the back.

The decision immediately appeared to have backfired when pacy Brazil striker Pato exposed their obvious lack of pace.

Straightaway Barca created a slew of chances for an equaliser before World Player of the Year Lionel Messi, after striking a curling freekick against the post, once again came to the rescue.

The Argentina forward, top scorer in the competition the past three seasons, danced through the Milan defence to the byline and squared for Pedro to rifle home with goalkeeper Christian Abbiati stranded.

Barca had recovered their customary poise and a goal had long been coming when Villa stepped up to crack the ball into the top corner from 30 metres.

Substitute Ivan Perisic volleyed in two minutes from time to hand Borussia Dortmund a deserved 1-1 draw against Arsenal in their Champions League Group F opener on Tuesday.

Arsenal looked to be cruising to an unexpected victory after taking a first-half lead through captain Robin van Persie but Perisic thundered in to rescue a draw.

The English club had to endure a barrage of attacks and were lucky not to concede a goal before captain Van Persie struck on 42 minutes, firing in after a defensive blunder by Dortmund captain Sebastian Kehl.

The hosts, 1997 Champions League winners and making their first appearance in Europe's premier competition in eight years, got off to a flying start in front of 65,000 fans but looked to have been punished for a string of missed chances before Perisic's equaliser.

With Arsenal coach Arsene Wenger watching the frantic start from the stands due to a suspension, each team missed a glorious early chance. The London side's Gervinho sneaked into the box but failed to chip the ball over keeper Roman Weidenfeller and in the ensuing quick break Kevin Grosskreutz fired over the bar.

Chelsea's annual pursuit of the continent's biggest club prize that has so far eluded them began in encouraging fashion for the latest coach tasked with the mission as Andre Villas-Boas's team beat Bayer Leverkusen 2-0 in Group E.

Chelsea welcomed back Michael Ballack to Stamford Bridge but it was to be no happy return for the German midfielder as goals by Brazil defender David Luiz and Spain forward Juan Mata gave Villas-Boas the perfect start to the campaign.

Curly-topped 24-year-old Luiz curled the ball around the German defence and into the far corner of the net after a superb 67th minute build-up involving Ashley Cole and Fernando Torres to put Chelsea ahead en route to a hard-fought victory in a tough group opener.

Juan Mata added the second with the last kick of the game in stoppage time to give Chelsea a rather flattering 2-0 victory.

A minute before Luiz scored, Leverkusen almost took the lead when former Chelsea favourite Michael Ballack advanced on goal with only Petr Cech to beat, but the Czech keeper blocked his shot.

Both teams had the ball in the net inside the opening four minutes, but both goals were disallowed by the officials.

Chelsea manager Andre Villas-Boas, in charge of his first Champions League match, left veterans John Terry and Frank Lampard out of the starting lineup, and his gamble paid off as a youthful looking Chelsea team picked up three valuable points.