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BUENOS AIRES, Argentina -- Boca Juniors suffered a shock 3-0 home defeat by San Lorenzo in the Argentine Clausura championship and their great rivals River Plate lost 2-1 to modest Argentinos Juniors on Sunday.
Humble Arsenal took full advantage by beating Velez Sarsfield 3-1 to go top with maximum points from their four games.
San Lorenzo, thumped 7-1 at home by Boca Juniors in their last meeting, stunned the Bombonera stadium when Diego Rivero broke down the right and crossed for Ezequiel Lavezzi to head in a goal after 23 minutes.
Boca playmaker Juan Roman Riquelme, who had an otherwise quiet game, hit the crossbar with a free kick before half-time but his side were in more trouble when Clemente Rodriguez was sent off four minutes into the second half.
Gaston Fernandez fired the second in the 66th minute and Andres Silvera rifled in the third from 12 metres five minutes later.
"The way we played was marvelous," San Lorenzo coach Ramon Diaz told reporters.
San Lorenzo, with 10 points from four games, moved up to second and Boca, with seven points, dropped to seventh.
River, who went into their match with three wins in three games, looked set to maintain their record when Marco Ruben fired through the legs of goalkeeper Marcelo Pontiroli in the 18th minute.
But Sebastian Carrera headed Argentinos level eight minutes later and Gabriel Hauche fired the winner eight minutes into the second half after the River defense failed to clear.
Shortly afterwards, play was delayed for nearly half an hour by a floodlight failure.
Second-half goals from Lucas Valdemarin and Carlos Castiglione gave Arsenal victory over Velez. The teams were level at halftime after Pablo Sebastian Garnier gave Arsenal an early lead and Mariano Uglessich levelled three minutes before the break.
Champions Estudiantes won 2-1 at Belgrano as 36-year-old Jose Luis Calderon scored a stoppage-time winner.
Pablo Piatti, 17, had given Estudiantes a first-half lead and Matias Oyola equalized in the 56th minute.
Jorge Burruchaga's Independiente continued their unhappy run as they lost 2-1 at Banfield despite taking a 34th-minute lead through Juan Eluchans.
Banfield's Javier Villareal and Carlos Salvatierra scored in the last 15 minutes to leave Independiente 19th with one point.
Independiente's arch-rivals Racing Club also remained winless after losing 1-0 at home to Newell's Old Boys.
Fernandez celebrates scoring the second goal for San Lorenzo.