My Brazilian friend Marina and I were picking up a visiting friend from New York, who heads an NGO, in her hotel lobby near Paulista, the most prestigious avenue in São Paulo. It was 7:30 on a busy Friday night last October.

We walked up to a taxi outside the hotel. I sat in the front to let the two women chat in the back. Marina asked me to Google the restaurant menu. I was doing so when I saw a teenage boy run up to the taxi and gesticulate through my open window. I thought he was a beggar, asking for money. Then I saw the gun, going from my head to the cell phone.

“Just give him the phone,” Marina said from the back seat.

I gave him the phone. He didn’t go away.

“Dinheiro, dinheiro!”

I didn’t want to give him my wallet. The boy was shouting obscenities. “Dinheiro, dinheiro!”

The boy’s body suddenly jerked back, as a man’s arm around his neck pulled him off his feet. The man, dressed in a black shirt, was shouting; he had jumped the boy from behind. He started hitting the boy. The taxi driver sitting next to me was stoic. He said that this had never happened to him before, but he couldn’t have been more blasé.

The next thing I saw was the boy and another teenager, probably his accomplice, running away fast up the street. The man in the black shirt chased them a bit, then came back panting to the taxi. “Did the bastard get anything?” our savior, whom we later nicknamed Batman, asked. He wasn’t a plainclothes cop, as I’d originally thought; he was just an ordinary citizen who was tired of the criminals.

“A phone,” Marina responded.

“Sons of whores. These motherfuckers—they always come in twos. Cowards.”

The taxi driver drove us to the nearest police station. Two lethargic cops were the only people there. “We get ten of these a day, just in this precinct,” said one of them.

The other cop went over to check in his register. “Three before you today.” There are 319 armed robberies a day in São Paulo.

Everyone in this country has a story. Priscilla, whom I met the next day, has been robbed ten times. Once a kid held a piece of glass from a broken bottle to her neck. Another time she was in a home invaded by gunmen, and one of them held a gun to her head for forty minutes.

I had gotten off lightly—just my phone taken. I still had my wallet, thanks to Batman, and I wasn’t beaten or killed or kidnapped.

The cities of Brazil are some of the most violent places in the world today. More people are murdered in Brazil than in almost any other country.…