Brazilian security forces have moved into a slum near Rio airport, as part of efforts to drive out drugs gangs before this year's football World Cup.
The vast Mare favela is considered to be one of the most dangerous drug-trafficking areas in the city.
Authorities have been carrying out a slum "pacification" programme aimed at making the city - which also hosts the 2016 Olympics - safer.
Large quantities of weapons were reportedly seized in Sunday's raid.
It began before dawn, when more than 1,000 police backed by soldiers and armoured vehicles took up position in Mare, one of Rio largest shantytowns.
The area - close to Rio's international airport - is home to about 130,000 residents.
The BBC's Julia Carneiro in Sao Paulo says it has become a hub for drug traffickers who have been pushed out of other communities by the pacification programme.
The authorities said the whole area was occupied within 15 minutes.
But in the afternoon, clashes erupted between rival groups and a 15 year-old boy was killed.
Residents said the unrest started with teenagers hurling stones at each other but ended in a shoot-out.
At least another three people were injured and taken to a hospital nearby.
Earlier, police had seized "large quantities of drugs and weapons" hidden near the Olympic Village, GloboNews TV reported.
The favela pacification programme was launched in 2008, when Rio launched a successful bid to host the 2016 Olympics.