It is 11:47. In civilian garb, Colonel Montagna's cousin peeks out from the corner between Raul Pompeia and Francisco Otaviano at the Coastal Artillery HQ. In 4 private cars behind him are Montagna and 19 other ECEME officers (a 20th got stuck in the elevator), with 2 INAs, a Thompson and 2 grenades. The cousin spots at the entrance a corporal with a Thompson. Some men on a hill wave to show them the path of least resistance, but he doesn't understand.
The cars speed down the street and before they stop the men jump out. Montagna punches the guard and grabs his Thompson. They charge into the building and through the flanks. Gunfire is exchanged and each side gets one wounded. Fighting extends to the Arpoador, the other peninsula to the SW. At 12:30 the last resistance surrenders. Montagna has seized the Coastal Artillery HQ for himself.

The whole experience was pointless. They took an administrative building, not the fort which was uninvolved. Apparently the fort's personnel already had controlled the situation inside the building, so attacking it was unnecessary and even friendly fire.
Yet the farcical battle was caught by the media, televised and acquired overblown publicity and fame.

The Fort then took two platoons, each with MGs and a 75 mm gun, to shield itself. One was in Avenida Atlântica facing the Duque de Caxias Fort and another at Francisco Otaviano against the 8th Motorized Coastal Artillery Group, which sent a platoon to reconquer Copacabana but was intimidated away. Civilian cars helped form the blockade. There were fears of an armored attack or an amphibious landing, which didn't materialize.