In a tense affair at the Estadio da Luz, both teams found it hard to create opportunities, though the host shaded the first half as it pressed to get its nose in front on home soil.

But a rare moment of attacking quality from Barella put Inter ahead, before substitute Lukaku scored from the spot to clinch the win for the Serie A side.

Simone Inzaghi's men are now very much in control heading into the second leg, set to be played at San Siro next Thursday.

The first half was a cagey affair with clear-cut chances few and far between, though Rafa Silva nearly punished a defensive mistake when he saw a blasted effort denied by Andre Onana after pouncing upon Federico Dimarco's loose header.

Francesco Acerbi also sent a fierce drive from range just over the crossbar as the game went into the interval goalless, with Inter managing just two touches in Benfica's box in the first half.

But the visitor would take the lead in the 51st minute with their first shot on target, Barella's delightful back-post header nestling in the bottom corner after Alessandro Bastoni's cross picked him out.

The host nearly levelled when Rafa saw a shot blocked before an almighty goalmouth scramble ended with Inter finally clearing.

Henrikh Mkhitaryan could, and perhaps should, have doubled his side's advantage after he was played in by Joaquin Correa, but his shot from a tight angle was well saved by Odisseas Vlachodimos.

Inter would add a second goal before the end, though, as Lukaku lashed home a penalty after Joao Mario handled Denzel Dumfries' cross to leave the Portuguese side up against it if it is to reach its first Champions League semi-final in 23 years.