Bayern Munich have opened up a 4-0 lead after the first leg of their Champions League semi-final tie against Barcelona.

Visiting goalkeeper Victor Valdes had to save from Arjen Robben as early as the fourth minute before the hosts had a penalty appeal turned away when Philipp Lahm's shot struck the elbow of Gerard Pique.

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Bayern took the lead in the 25th minute when Robben's looping cross was headed back across goal by Dante for Thomas Muller to stoop low and head into the net.

The visitors' best chance of the half came when Pedro's low cross was turned just wide by Dante, with Lionel Messi lurking just behind.

Bayern were 2-0 ahead just four minutes into the second half when Mario Gomez volleyed home from a couple of yards from Muller's knockdown. The German frontman did appear to be offside when Muller made contact.

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The German side continued to threaten as Muller and Franck Ribery fired wide before Robben got above Alexis Sanchez but couldn't head Bastian Schweinsteiger's free kick on target.

Barca could have snatched a vital away goal midway through the second half, but young defender Marc Bartra couldn't poke a loose ball past Manuel Neuer from a couple of yards.

Just a minute later it became 3-0. Robben did a stepover to beat Jordi Alba who was then barged off the ball by Muller to allow Robben a free shot at goal and he nestled it in the far corner.

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Barca defender Bartra again had a clear opening to grab a goal but blazed high and wide after being left unmarked from a free kick.

Muller completed the rout for Bayern in the 82nd minute by bundling David Alaba's cross into the net after some swift interplay.

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Frustration was creeping into the Barcelona ranks and Alba will miss the second leg because of his booking for throwing the ball into Robben's face.

The return leg takes place next Wednesday evening at Camp Nou.