Robert Lewandowski’s incredible goalscoring form continued as the striker’s hat-trick helped Bayern Munich thrash Dinamo Zagreb 5-0.

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The Croatian champions, who beat Arsenal in their opening match, went into this clash on a 45-game unbeaten run stretching back to last November but that was brought to a shuddering halt at the Allianz Arena.

Bayern took the lead when Costa smashed a low shot beyond Eduardo at his near post.

Guardiola’s men doubled their lead after 21 minutes thanks to Lewandowski.

Thiago Alcantara took advantage of sloppy defending and slipped the ball to Lewandowski, who tapped in from close range.

Bayern kept coming and bagged a third goal after 25 minutes. Gotze played a neat one-two with Kingsley Coman and the German’s strike beat Eduardo, who failed to get a firm hand to the ball.

Things got a whole lot worse for the visitors when Lewandowski struck again to make it 4-0. He met Costa’s corner with a header which crashed into the net via the crossbar.

The phenomenal Lewandowski completed a hat-trick 10 minutes after the interval. Alcantara flicked the ball through to the Pole and he scooped his effort over the helpless Eduardo and into the net.

Arsenal’s hopes of qualifying from Group F lie in tatters after a dismal 3-2 loss at home to Olympiacos.

The Greek side had lost on all 12 of their previous trips to England but made it lucky 13 as stand-in Gunners goalkeeper David Ospina had a night to forget.

Boss Arsene Wenger had warned his side already faced a must-win tie following their opening loss at Dinamo Zagreb but they fell behind after 32 minutes when a low shot from Felipe Pardo deflected off Alex Oxlade-Chamberlain.

Although Theo Walcott netted a quick equaliser, Arsenal found themselves trailing again when Ospina, chosen ahead of Petr Cech, dropped a corner over the line.

Alexis Sanchez levelled on 65 minutes, but Arsenal hit the self-destruct button once more as Olympiacos immediately scored through substitute Alfred Finnbogason.

Chelsea, who have made a poor start in their attempt to retain their Premier League title this season, crashed 2-1 at Jose Mourinho’s former club Porto.

They fell beind when Yacine Brahimi waltzed inside Branislav Ivanovic and behind Pedro, then fired in a shot which keeper Asmir Begovic punched only as far as Felipe Andre Andre, who hooked the ball in.

Willian replied with a delightful curling free-kick on the stroke of half-time, but then woeful defending allowed Maicon to head in a corner seven minutes into the second half.

Diego Costa hit the crossbar, but Porto placed Chelsea under siege and the visitors were grateful to Begovic for some crucial saves while a Danilo header came off the post.

The shocks did not stop there with Italians Roma stunned 3-2 by Belarussian outsiders BATE Borisov.

The hosts raced into a 3-0 lead with just half an hour gone thanks to an early goal from Ihar Stasevich and Filip Mladenovic’s double.

Gervinho and Vassilis Torosidis pulled goals back in the second half but BATE held on for a famous win.

Barcelona averted another upset with two late goals to some from behind and beat Bayer Leverkusen 2-1.

Kyriakos Papadopoulos headed the Germans into a 21st-minute lead and Barca, without talisman Lionel Messi through injury, struggled to get back into the game.

But Carnicer Sergi Roberto eventually equalised in the 81st minute and 90 seconds later Luis Suarez secured three points for the off-colour Catalan giants.