United were second best for much of the game, though the nearest the hosts came to breaking the deadlock was a Samuel Eto'o slice into the sidenetting and two late Thierry Henry efforts.
The result leaves next week's second leg at Old Trafford wide open, with both sides entitled to feel relatively happy with the scoreline.
But it could have been so much better for United, who saw Ronaldo's first-minute free-kick deflected over for a corner. From the resulting flag kick, Ronaldo was first to react but his goal-bound header was blocked by the raised hands of Argentinian defender Milito with referee Massimo Busacca pointing straight to the penalty spot.
However, the usually-reliable Portuguese winger lost his nerve and fired his spot-kick wide of Victor Valdes' goal.
Barca made a bright start to the second period with Messi seeing a shot deflected wide and Gianluca Zambrotta firing another effort just over.
Eto'o then skipped around Rio Ferdinand's challenge in the area, but his cut-back was cleared by Owen Hargreaves. Moments later, the Cameroon striker missed a great opportunity when some slick passing carved United open, but his shot hit the sidenetting.
As play switched down the other end, Michael Carrick was unlucky to see his left-footed effort also flash the wrong side of the net with Valdes looking beaten.
Andres Iniesta then saw his theatrical attempts to win a penalty waved away by the referee, while the late introduction of Henry from the bench failed to break the deadlock despite the Frenchman seeing a stinging drive parried by van der Sar and another effort, from a free-kick, comfortably saved.