Helton howler helps PSG clinch top spot
Argentine forward Lavezzi's low shot slipped under Helton's body in the 61st minute as PSG leapfrogged Porto into first place with 15 points from six games, two ahead of the 2004 champions.
"We produced a good performance. We needed to regain this footballing quality and this attitude," PSG coach Carlo Ancelotti told a news conference after his side bounced back from losing three of their previous five games in Ligue 1.
"The players showed a lot of pride tonight because the critics have been very harsh. Now we can forget the Champions League for a while and focus on Ligue 1."
PSG and Porto had already secured their places in the last 16.
Ancelotti's team went ahead in the 29th minute when Thiago Silva nodded in a Maxwell free-kick but Porto levelled four minutes later with a far-post header from Jackson Martinez ending a flowing move.
PSG had started the match strongly and kept the Portuguese side quiet in the early stages.
Zlatan Ibrahimovic went close for the home team in the second minute after excellent work from Jeremy Menez.
Porto began to get a foothold in the game midway through the half with striker Martinez proving a handful for centre back Thiago Silva.
Ibrahimovic missed another chance when his far-post volley went wide before Thiago Silva opened the scoring.
Martinez equalised after a good cross by Danilo only to see Lavezzi grab the winner with his third goal in two Champions League matches.
Porto almost snatched a second goal in stoppage time when James Rodriguez headed just wide.
"I am not satisfied," said Porto coach Vitor Pereira. "We were far more aggressive when PSG visited us," he added, referring to his team's 1-0 victory in October.
"This is what we lacked tonight. We regained it when we went 2-1 behind but unfortunately we did not manage to get an equaliser."