Nearly 30 million people have died of AIDS since it was first identified in 1981. In South Africa, a nation of 50 million people, nearly six million people are living with HIV.
Across the nation, people recommitted to trying to end the disease.
President Obama announced $50 million will go to HIV and AIDS treatment in the U.S. The White House also displayed a red ribbon to mark the day.
Health officials in the nation's largest city recommended that any residents living with HIV be offered AIDS drugs as soon as the virus is diagnosed, an aggressive move that has been shown to prolong life and stem the spread of the disease.
In New York, several protesters calling for more funding to fight AIDS were arrested after forming a human chain and lying on the pavement near New York's City Hall. The protesters were part of a larger group of about 100 people who marched from lower Manhattan's Zuccotti Park to City Hall. Some Occupy Wall Street activists joined the march.
The protesters want Mayor Michael Bloomberg and members of Congress to institute taxes on Wall Street trade to pay for AIDS treatment.
In Los Angeles, the AIDS healthcare foundation and Magic Johnson were set to hold several free HIV testing and awareness events.
The goal is to get AIDS drugs to 2 million more people living with HIV by the end of 2013.
The Associated Press contributed to this report.