"It's a way to remind the population that AIDS has yet to be cured and that condoms are the only way, technically speaking, to fight the AIDS virus and other sexually transmitted diseases," said Carlos Tufvesson, Rio de Janeiro's sexual diversity coordinator.
On Saturday, Rio will offer tests to the public to detect the HIV and syphilis viruses at 185 health centers in the city. Results will be provided within 10 days.
Rio's government has invested two million reais ($1.1 million) in the program, the largest city-wide campaign in Brazil seeking to combat ignorance, prejudices and misinformation about AIDS.
An estimated 250,000 Brazilians live with the HIV virus that causes AIDS without knowing it. But Brazil has successfully stabilized the pandemic within its borders, recording a 0.61 percent drop in new cases from 2009 to 2010 according to Health Ministry figures.
Authorities still remain concerned about the rise in the number of cases in homosexual males aged 15 to 24 -- from 25.2 percent in 1990 to 46.4 percent in 2010 -- especially in transvestites.
The red lighting will last three days in Rio. A number of other countries are also participating in the campaign, including Argentina, Australia, Britain, Canada, China, Ireland, South Africa and the United States.
In downtown Buenos Aires, a giant red flag was draped over the Obelisk, while a group of activists handed out condoms to passers-by. The Casa Rosada government palace was decorated with a big red bow.
About 130,000 people live with HIV in Argentina, and two thirds of them do not know they carry the virus, according to activist groups.
Nearly two million Latin Americans live with HIV of whom 950,000 require antiretroviral medications, though only 478,000 have access to the drugs, according to the UNAIDS program.
In 2009, over 100,000 people in the region contracted HIV, while 70,000 died of AIDS-related complications.