Barcelona had to scrap for a 1-1 draw at Olympique Lyon and Manchester United were frustrated in a goalless game at Inter Milan as the Champions League returned with a lack of respect for reputation on Tuesday.

While Barcelona needed an untidy goal from Thierry Henry to take a slim advantage from the first leg of their last-16 tie against Lyon, holders United failed to take advantage of a series of early chances in Milan.

United did extend their unbeaten run to a record 20 Champions League matches but manager Alex Ferguson again failed to get the better of Jose Mourinho and they will have to tread carefully in the second leg at Old Trafford.

The other two ties are also well poised after Porto twice came from behind to draw 2-2 at Atletico Madrid and Arsenal produced the only victory of the night, 1-0 at home to Roma thanks to a Robin van Persie penalty.

JITTERY START

Barcelona's elegant passing game, similar to the one that took Spain to victory at Euro 2008, has been the envy of Europe this season but their form has deserted them lately and they made another jittery start against Lyon.

The French side took the lead from a source familiar to anyone following the Champions League down the years, as the game's leading set-piece specialist Juninho deceived Victor Valdes with a dipping free kick from the left.

Barcelona were terrible in the first half and Karim Benzema came within a whisker of making it 2-0 but they improved after the break and got a deserved equaliser when French striker Henry stooped to head in a deflected corner.

The away goal gave Barcelona, twice former champions, a slim advantage to take to the Nou Camp in two weeks.

"We missed our chance in the first half, when we should have been rewarded for our efforts but we gave everything," said Lyon coach Claude Puel. "(In the return leg) we will have opportunities."

MISSED CHANCES

The two English sides in action on Tuesday may also have cause to rue missed opportunities.

United were last beaten in a Champions League match when they lost 3-0 away to AC Milan in 2007 but a similar result from their latest trip to the San Siro never looked on the cards.

Cristiano Ronaldo forced a point-blank save from Inter keeper Julio Cesar and the chances came thick and fast for United for the remainder of the first half.

Mourinho, whose Porto side knocked out United on their way to winning the Champions League in 2004, inspired a huge improvement in the second half but United came desperately close to a winner when Ronaldo's last-gasp free kick was beaten away.

Arsenal were the only winners but it should have been a more commanding victory.

They had to be content with the penalty from Van Persie, which the Dutchman won himself when he was bundled over by French defender Philippe Mexes after 37 minutes.

Goals were plentiful in Madrid, where Atletico and twice former champions Porto fought out a thriller.

Atletico hit the ground running, Maxi Rodriguez giving them the lead in the third minute before Lisandro Lopez burst through to put the Portuguese side level.

A mistake from Porto keeper Helton let Diego Forlan's weak shot slither in at the end of the first half but back Porto came again with Lopez turning in Aly Cissokho's low cross.

The remaining four first-leg matches in the first knockout round are being played on Wednesday, with nine-times champions Real Madrid at home to Liverpool in the outstanding game.

Chelsea, beaten by United in last season's all-English final, are at home to Juventus, Villarreal host Panathinaikos and Sporting are at home to Bayern Munich.