Plans to ring two slums in Rio de Janeiro with a three-metre (10-foot) wall have been condemned by human rights groups and the city's own mayor.
Officials in the Brazilian city are pushing the idea after a drug-related turf war at the weekend left two policemen and six others dead.
"We need to build it immediately," said state Deputy Governor Luiz Paulo Conde.
Amnesty International said the wall would penalise innocent people and was unlikely to be effective.
Another group, Global Justice, said the barrier would create "social apartheid" when what was needed was investment in poor communities.
However, the authorities in Rio state say the wall will help the security forces control the Rocinha and Vidigal favelas, or slums, where 1,200 officers have been on patrol since Friday.
Drug traffickers from Vidigal are believed to have tried to seize control of drug and arms trade.
"The wall isn't to stop the violence, it is to mark off territory," Mr Conde said.
'Theme park'
Rocinha has a population of about 150,000, making it one of Latin America's largest shanty-towns, and both it and Vidigal overlook the city's richest districts and most fashionable beaches.
It had until recently been relatively peaceful and tourists felt able to visit while local businesses thrived.
But hotel guests in the city were shocked to see red and blue tracer bullets streaking across the night sky at the weekend, local media report.
Rio Mayor Cesar Maia said the wall plan was "unbelievable".
"They want to create some sort of theme park on drug and cocaine trafficking," he said.
However, Deputy Governor Conde said the wall would also play a valuable environmental role, protecting forests from the slums' expansion.
"We won't allow unlimited expansion in the name of drug-trafficking," he said.
Police believe that about 40 heavily armed drug gang members escaped their searches on Sunday by hiding out in the forest.
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