This part of the Amazon, once thought to be virgin forest, has in fact been touched by extensive human activity.
Researchers found traces of a grid-like pattern of settlements connected by road networks and arranged around large central plazas.
There is also evidence of farming and wetland management, including possible remains of fish farms.
The settlements are now almost completely overgrown by rainforest.
The ancient urban communities date back to before the first Europeans set foot in the Upper Xingu region of the Brazilian Amazon in the 15th Century.
Professor Mike Heckenberger, from the University of Florida, in Gainesville, said: "These are not cities, but this is urbanism, built around towns."
"They have quite remarkable planning and self-organisation, more so than many classical examples of what people would call urbanism," he said.
However, they only held around 50,000 people, compared with the 13 million in LA.
The extraordinary conclusion is reached by anthropologists from the University of Florida and Brazil, and a member of the Kuikuro, an indigenous people who are the descendants of the settlements' original inhabitants.
"If we look at your average medieval town or your average Greek polis, most are about the scale of those we find in this part of the Amazon," said Prof Mike Heckenberger of the University of Florida, lead author of the paper published today in the journal Science.
"Only the ones we find are much more complicated in terms of their planning."
If you find it somewhere else, it has to be something else," Prof Heckenberger said. "They have quite remarkable planning."
But these findings should blow up the Hollywood impression that Amazon Indians were and are so primitive that they couldn't accomplish anything. That if the ancients created a complex civilization, they couldn't have been Indians.
For more on the subject, see Indiana Jones and the Stereotypes of Doom.