UEFA Champions League: Arsenal, Chelsea Crash as Bayern Munich Cruise

Arsenal and Chelsea had a Champions League night to forget on Tuesday as a Robert Lewandowski hat-trick helped Bayern Munich crush Dinamo Zagreb.

And champions Barcelona, with injured icon Lionel Messi watching from the stands, followed up their opening Group E draw at Roma with a stirring come-from-behind 2-1 win against Bayer Leverkusen.

The Germans appeared to have the three points in the bag after Kyriakos Papadopoulos's header before the break.

But Luis Suarez turned the game around in the final 10 minutes as Sergi Roberto tapped home the rebound after the Uruguayan's initial effort had been saved.

And the former Liverpool striker thumped home a stunning effort to hand Barca a hard-fought three points, two minutes later.

"We couldn't play great football, but we had to win the game. We had to show we are the champions," said Suarez.

There was a sting in the tail as captain Andres Iniesta hobbled off after an hour.

With Roma sent on their way 3-2 by BATE Borisov it left Barca shading Group E.

Jose Mourinho's return to Porto, the team who the Portuguese guided to the 2004 title, proved an unhappy affair.

Andre Andre's opener for Porto late in the first half was quickly cancelled out by a superb Willian free-kick, but Maicon's 52nd-minute header proved to be the difference in this Group G clash played at a rip-roaring pace.

Dynamo Kiev, 2-0 winners over Maccabi Tel Aviv, top the table on goal difference with Chelsea in third.

It was a good performance with two ridiculous mistakes," said Mourinho.

"The second goal is a really bad mistake. We watch dozens of repetitions of Porto taking corners. I thought we were completely ready for that.

"But apart from that it was a balanced performance. The defenders played well and had no problems, but two ridiculous mistakes and we were punished."

Arguably the most stunning result of the evening emerged from the Emirates Stadium, where Greek champions Olympiakos shocked Arsenal 3-2 to leave the Gunners stone last in Group F.

The Greeks, ending a 12-match losing run in England, left pointless Arsenal and their coach Arsene Wenger ashen-faced, and the team's fans jeering at the final whistle.

After falling behind to Felipe Pardo's deflected opener, Arsenal equalised through Theo Walcott.

But then a horrendous mistake from goalkeeper David Ospina, selected over regular first choice Petr Cech, allowed a Kostas Fortounis corner to slip through his grasp for an embarrassing own goal.

Alexis Sanchez dragged Arsenal back into it with a second half equaliser, but incredibly the north Londoners imploded again with more slack defending gifting Iceland striker Alfred Finnbogason the winner.

"It is still possible to qualify but it is difficult to swallow losing a game like that," Wenger said.

"We lost it on a lack of defensive concentration and bad luck. They had four shots on goal and we conceded three goals.

"A goalkeeper (Ospina) can make a mistake. That doesn't explain that we lose the game. It happened to Petr Cech already this season."

Beaten in the group curtain raiser in Zagreb Arsenal are now up against it to extricate themselves from the pool stages.

To compound their plight free-shooting Bayern Munich await, the German giants flexing their muscles in Zagreb.

Bayern top the table, Pep Guardiola's side cruising towards the knockout stages after Lewandowski's treble, and goals from Douglas Costa and Mario Goetze steamrolled Dinamo Zagreb 5-0.

"We played in a very concentrated fashion from the start and when you take your goal chances, the game was over after 30 minutes," said Bayern captain Philipp Lahm.

Poland hot-shot Lewandowski has now scored 10 goals in his last three games after netting five in just nine spectacular minutes of Bayern's 5-1 hammering of Wolfsburg last Tuesday before claiming two more in Saturday's 3-0 win at Mainz.

"He's had an unbelievable week behind him," admitted Lahm.

Elsewhere, Zenit St Petersburg remained unbeaten with a 2-1 home win over Belgian outfit Gent to top Group H from Valencia, 1-0 winners at Lyon.

Wednesday's menu features the two Manchester clubs, United and City, against Vfl Wolfsburg and Borussia Moenchengladbach, while Real Madrid are in Malmo and Paris Saint-Germain have a tricky assignment away to Shakhtar Donetsk.