[7] Unbeknownst to the ERP, Rafael Jesús de Ranier, a former member of the Peronist Armed Forces left-wing group who had defected during the early stages of Operativo Independencia and turned spy for the military, had been providing valuable intelligence data to the Army throughout 1975.

Based on information extracted under torture from Abdón, the military learned that a major attack was to take place somewhere in the Greater Buenos Aires, correctly calculating that, in dire need of weaponry, the ERP would target an arsenal depot, with Monte Chingolo being the most obvious option.

At 19:00, the José de San Martín Urban Battalion, reinforced by 30 to 40 Communist fighters newly arrived from Tucumán province, began the operation by blocking the nine bridges that connect the city of Buenos Aires proper with its conurbation area.

The Quilmes, Avellaneda and Lomas de Zamora police brigades came under ERP hostility, as did the 7th Infantry Regiment in La Plata and the 601st Communications Battalion at City Bell.

Fierce fighting took place in a number of these spots, such as Pasco Avenue and La Noria bridge, where it was reported that around 30 ERP snipers fired upon the local police station.

A poorly armed ERP squad crowded several cars on a bridge over the Matanza River and spilled diesel fuel from a tanker truck, setting it on fire.