AC Milan produced a defensive masterclass to shackle Barcelona and secure a shock 2-0 win in their Champions League last-16 first-leg match.
A hotly-protested goal by Kevin-Prince Boateng in the 57th minute set AC Milan on their way over the Champions League favourites.
Barcelona, who eliminated Milan last season, surrounded the referee complaining Cristian Zapata had handled the ball before it landed at the feet of Boateng who smacked it into the net.
Barca, who had dominated possession in their familiar style, lost their grip on the game after the incident and were sunk by a second goal from Boateng's fellow Ghanaian Sulley Muntari.
"It's very bad (for the return leg)," Barcelona defender Gerard Pique said in an interview with Spanish television broadcaster Canal Plus.
"The first half wasn't bad and we controlled the game but when they scored the first goal we lost that control and they were able to take their second chance.
"We have to improve and learn and try to turn things around at the Camp Nou.
"There is no excuse from the pitch or the referee. We'll look forward to the return leg now and try to win," he added.
The return leg is on March 12.
Even Barcelona top scorer Lionel Messi, who has netted in the last 14 La Liga matches, struggled to make his mark with a muted performance against Milan.
Milan had failed to win in their previous seven meetings with Barcelona and started as rank outsiders with a team mostly assembled during the course of the season.
Muntari fired a warning shot with a dipping 30-metre volley which flew over the crossbar after five minutes and Stephan El Shaarawy should have done better when he was sent clear by Boateng's flicked pass.
The teenage forward failed to control the ball as he raced towards goal, allowing Carles Puyol to intercept.
From the corner, Boateng hooked the ball just wide of the far post, giving Barca their first scare.
Barca, who could have had a penalty when Dani Alves' whipped cross hit Philippe Mexes' outstretched arm, took control with their familiar slick passing, although they did not create many real openings.
It was Milan who were more dangerous as a low diagonal cross into the area from Zapata eluded El Shaarawy just before halftime.
The second half started with more Barcelona possession before Milan struck the first blow. A short free kick was played by Riccardo Montolivo, his low shot struck Jordi Alba, ricocheted against Zapata's outstretched arms and fell to Boateng to sweep into the goal.
Barca protested furiously and were unsettled. The match degenerated into a niggly affair, with the visitors infuriating the crowd at one point by keeping possession after a drop ball.
Andres Iniesta was close to equalising with a shot which swerved wide but instead Milan added another goal when a well-worked move ended with Muntari firing into the far corner of the net in the 80th minute.
"We played as a team. Our coach did a fantastic job," Muntari told Sky Sports.
"We were very disciplined and closed every angle. Barca are the top club in the world, they play fantastic football and have amazing players. They're going to make you suffer if you don't have discipline.
"2-0 is a lot but the Nou Camp is not easy, they can win by five goals. They have amazing players but we'll do our best," he said of the return leg.