Lopez double pulls Porto level at Atletico
Dangerous forward Hulk caused problems for the home defence as Porto came back strongly and he blazed a shot over the bar before Forlan stung Helton's hands with a powerful drive.
The equaliser came after 22 minutes when defender Pablo Ibanez badly misjudged a header and Lopez raced through to drive the ball in off keeper Leo Franco.
A dreadful mistake by Helton gifted Atletico their second goal in first-half stoppage time. Forlan fired a speculative long-range effort and the ball squirmed through the Porto keeper's hands and into the net.
Porto almost equalised immediately after the restart when Franco saved superbly from a Cristian Rodriguez header and Lisandro spooned a first-time shot over the bar with only the keeper to beat.
It was only a matter of time before the visitors got a deserved second and Lopez turned in Aly Cissokho's low cross with 18 minutes left to make it a disappointing end to new Atletico coach Abel Resino's Champions League debut.
The second leg is at Porto on March 11.