Barca lost by a two-goal margin for the first time under manager Pep Guardiola as strikes from Wesley Sneijder, Maicon and Diego Milito, after Pedro had given Barca the lead, set up a fascinating second leg at the Nou Camp next Wednesday.
Barcelona captain Carles Puyol was booked and will miss the return along with Inter midfielder Dejan Stankovic.
In one of the most electric atmospheres inside the San Siro for years, both sides stretched every last sinew to the limit as Europe's top club competition again showed why fans the world over are so hooked on the excitement.
"Anything can happen in the second leg. We can go there and lose the game and lose the coach," Inter boss Jose Mourinho cryptically told a news conference.
"What happened today happened, it wasn't a dream. It was reality. We deserved to win. We saw a team working until the death. The team was destroyed by what they gave on the field."
The 22-year-old was again preferred to Thierry Henry in Barca's starting line-up and the diminutive livewire forward is fast looking a good bet to gatecrash Spain's World Cup squad.
"I congratulate them but we'll see them next week," said Guardiola, whose side beat Inter 2-0 at home in the group stage.
"We are in the semi-final of the Champions League against a great team, it can happen. We move forward."
Ex-Inter striker Zlatan Ibrahimovic, booed repeatedly by his former beloved crowd, had a few sniffs at goal while Lionel Messi only had one good effort after largely being kept under wraps by the sheer brute force of Inter's defen
