Eder Militao's own goal handed a depleted Barcelona a surprise 1-0 away victory over Real Madrid in a stormy Copa del Rey semi-final first leg on Thursday.
The Catalans arrived on the back of two consecutive defeats and without an injured trio of key players – top scorer Robert Lewandowski, key midfielder Pedri and explosive winger Ousmane Dembele.
After their Europa League elimination by Manchester United last week and a first ever loss against La Liga minnows Almeria, Madrid fans smelled blood, but left bitterly frustrated.
Militao deflected a rebound into his own goal after 26 minutes to split the teams and hand Barcelona the edge ahead of the second leg on April 5 at Camp Nou.
Barcelona lined up with Marcos Alonso out of position at centre-back, after Andreas Christensen was only deemed fit enough to start on the bench.
Sergio Busquets made his 46th Clasico appearance, overtaking former Real Madrid and Barcelona greats Sergio Ramos and Lionel Messi to set a new record.
"We knew how to compete, the team did a great job physically, we were very solid, we blocked them through the middle," Barca captain Busquets told TVE.
"Even though they had the ball and possession, they didn't have clear chances and we had the goal, a small advantage to take for the second leg."
Despite Barcelona's seven-point lead over champions Madrid in La Liga, recent results and performances placed Los Blancos as favourites to triumph in the first leg, at least according to Barca coach Xavi Hernandez.
Madrid have not won the trophy since 2014 and pushed Barca back early on, with Karim Benzema scoring, but the goal was disallowed for a clear offside against the French striker.
A handful of Madrid fans had protested outside the Santiago Bernabeu about Barca's involvement in the "Negreira case" – the Catalan club paying a former refereeing chief for advice until 2018.
The temperature rose considerably inside the stadium, when Vinicius Junior was booked midway through the first half for wrestling Frenkie de Jong to the ground.
Vinicius was fortunate not to see another yellow card as he remonstrated vociferously with the referee, and Madrid may still have been distracted by the argument moments later when Barcelona took the lead.
The goal came against the run of play, created by Ferran Torres, who slipped through Franck Kessie, just onside.
Thibaut Courtois saved the Ivorian midfielder's low shot but the ball rebounded off Militao, into his own net, with Nacho trying and failing to hack it clear.
Madrid players and supporters reacted angrily as Barcelona midfielder Gavi got away with smashing into Militao, but the hosts struggled to carve out chances.
Dani Carvajal lashed a volley high over the bar as Barcelona held their lead until the break, with supporters whistling the officials as they walked towards the tunnel.
With Carlo Ancelotti's side unable to muster a shot on target against an underwhelming Barcelona side, perhaps some of the home supporters' frustration aimed at the referee was misplaced.
Marc-Andre ter Stegen clawed away a Vinicius cross in the second half as Madrid struggled to connect with Benzema, isolated and unable to influence the game.
Record Copa winners Barcelona should have doubled their lead with 20 minutes remaining when Torres cut the ball back to Kessie, but his goal-bound shot was blocked by his own team-mate, Ansu Fati.
Rodrygo whipped a shot wide for Madrid in the final stages as they remained unable to test Ter Stegen, despite Barcelona's deficiencies.
More traditional roles were reversed as Madrid had more of the ball but Barcelona defended solidly and took the advantage back home.