Events have been held to mark the 20th World Aids Day, many highlighting the need to fight continued prejudices against people with HIV/Aids.
The UN recently reduced its estimate of the number of people with HIV/Aids, but the figure still stands at 33 million.
Campaigners warn that better treatment has bred complacency about the risks.
"The trend is encouraging but still for every person receiving treatment four others are newly infected," said former South African President Nelson Mandela.
'Breaking the cycle'
Mr Mandela was speaking at a concert in Johannesburg, South Africa, where international musicians, including Annie Lennox and Peter Gabriel, were performing alongside local talent.
"If we are to stop the Aids epidemic from expanding, we need to break the cycle of new HIV infections. All of us working together with government, communities and civil society can make the difference that is needed," he added.
The concert was the latest event organised by the 46664 campaign - which is named after Mr Mandela's old prison number and began five years ago.
South Africa has more HIV infections than any other country - five million South Africans are HIV positive.
After years in which the South African government was accused of "Aids denial", the country now has the world's largest programme of antiretroviral treatment.
But Aids campaigners say more must still be done - especially in preventing mother-to-child HIV transmission.
UN chief Ban Ki-moon said strong leadership was needed if the fight against the disease was to be won.
And he called for renewed efforts to focus on helping women, who now make up half of those living with Aids worldwide, and for better funding for prevention and care programmes.
"I call for leadership among all governments in fully understanding the epidemic - so that resources go where they are most needed," he said in a speech in New York.
"And I call for leadership at all levels to scale up towards universal access to HIV prevention, treatment, care and support by 2010," he added.
US fund increase
US President George W Bush on Friday urged the US Congress to authorise the doubling of financial aid to combat HIV/Aids to $30bn (£15bn) over the next five years.
"We will turn the tide against HIV/Aids once and for all," he said, adding that he would visit Africa during 2008.
In China, the Miss World pageant is coinciding with World Aids Day
Almost three-quarters of Aids-related deaths during 2006 were in sub-Saharan Africa. Two-thirds of those living with HIV are found there.
But the number of people living with the virus has increased everywhere, with the most striking increases in East Asia and Central Asia/Eastern Europe.
The BBC's David Loyn, in Afghanistan, says that nearly 30 years after HIV/Aids first emerged onto the world stage, it is now moving into the country with unpredictable consequences.
Only 266 cases of HIV/Aids have been recorded in Afghanistan but returning refugees, truck drivers, and Afghans now flying abroad to work are bringing in the disease.
There are fears the official figures are just the tip of the iceberg, our correspondent says.
In India, the Red Ribbon Express, a train with a giant Aids ribbon running the length of its side, has set off on a journey of almost 27,000km (16,000 miles) to spread advice about tackling the disease to almost 60,000 villages across the country.
An estimated two and a half million people in India are affected by the Aids virus - only Nigeria and South Africa have higher numbers.
In Sanya, China, the 57th Miss World beauty pageant is being held to coincide with World Aids Day in an effort to raise awareness.
In India, the Red Ribbon Express is spreading anti-Aids advice
And Chinese President Hu Jintao has appeared on the front page of major state-controlled newspapers shaking the hand of a woman with HIV.
The head of the Roman Catholic Church, Pope Benedict XVI, has said this week that he is spiritually close to victims of Aids and their families, but the Vatican still opposes the use of condoms as a means of fighting the epidemic.
At his weekly general audience the pope called for stepped-up efforts to stop the spread of the HIV virus that causes Aids and deplored the disdain with which Aids sufferers are often treated.
Yet the Vatican refuses to end its opposition to the use of condoms.
Roman Catholics in many parts of the world, however, believe condom use helps save lives, according to an opinion poll published by a liberal Catholic group in the United States, Catholics for Choice.
According to the survey, a majority of Catholics living in Ghana, Ireland, Mexico, the Philippines and the US all believe that Catholic hospitals and government-funded clinics should be required to include condoms as part of Aids prevention.