Chelsea's Champions League hopes hang by a thread after Barcelona condemned them to defeat at Stamford Bridge.
Eto'o wheels away after scoring Barca's crucial second goal
Asier del Horno was sent off after 36 minutes for a foul on Lionel Messi - but Thiago Motta's own goal gave Chelsea the lead after the break.
Barcelona pinned Chelsea back and John Terry headed Ronaldinho's free-kick past Petr Cech with 20 minutes left.
Terry cleared off the line from Henrik Larsson and Ronaldinho before Samuel Eto'o headed home after 80 minutes.
Barcelona's attacking brilliance deserved the victory, and now Jose Mourinho's side face a massive task to turn the tie around in the second leg at the Nou Camp.
Chelsea had little joy against the Barcelona defence in the opening 45 minutes - while the Catalan side posed all the serious threats.
Terry had to produce a desperate block to deny Ronaldinho in the 16th minute.
And the Brazilian almost broke the deadlock on the half-hour when Chelsea goalkeeper Cech had to dive smartly to his left to save his snap shot.
The game exploded on 36 minutes when Del Horno received his red card.
Messi escaped the attentions of Arjen Robben, but was halted by a clumsy challenge by Chelsea's Spanish defender.
The Argentine appeared to take a look at referee Terje Hauge before rolling around theatrically - and, when a melee involving players from both sides had been dispersed, Del Horno was sent off.
Chelsea boss Mourinho immediately sent on Geremi for Joe Cole, and also replaced the injured Hernan Crespo with Didier Drogba at the start of the second half.
It sparked Chelsea's most impressive spell of the game and they took the lead after 58 minutes.
Frank Lampard's free-kick was aimed for Terry, who unnerved Motta into turning the ball past Victor Valdes into his own net.
Chelsea had an escape three minutes later when the dangerous Messi raced into the area and fired across the face of goal with no Barcelona striker able to apply the finishing touch.
Barcelona introduced Larsson for Motta after 65 minutes as they attempted to put Chelsea under pressure.
And the Swede had a chance to equalise within seconds of his arrival, only to head over from Ronaldinho's cross.
Barcelona levelled with 20 minutes left - and this time it was Terry suffering the agony of scoring an own goal.
Ricardo Carvalho needlessly fouled Larsson, and Ronaldinho's swerving free-kick glanced off Terry's head and flew over Cech.
Messi had been Barcelona's outstanding performer, and he almost put them in front with a stunning curling drive after 72 minutes that rebounded to safety off the bar.
Chelsea were on the rack and it took two goal-line clearances from Terry to deny Larsson and Ronaldinho, while Cech also saved well from substitute Silvinho.
Drogba almost broke the shackles, but his touch was poor and Valdes saved.
And there was no escape from rampant Barcelona with 10 minutes remaining, with Rafael Marquez's cross finding Eto'o, who rose above Paulo Ferreira at the far post to seal the win.
Chelsea: Cech, Paulo Ferreira, Ricardo Carvalho, Terry, Del Horno, Makelele, Joe Cole (Geremi 40), Gudjohnsen, Lampard, Robben (Wright-Phillips 78), Crespo (Drogba 45).
Subs Not Used: Cudicini, Maniche, Duff, Huth.
Sent Off: Del Horno (37).
Goals: Motta 59 og.
Barcelona: Valdes, Oleguer, Marquez, Puyol, Van Bronckhorst (Sylvinho 69), Messi, Deco (Iniesta 84), Edmilson, Motta (Larsson 65), Eto'o, Ronaldinho.
Subs Not Used: Jorquera, Belletti, Ezquerro, Van Bommel.
Booked: Puyol, Iniesta.
Goals: Terry 71 og, Eto'o 80.
Ref: Terje Hauge (Norway).