The 18th International AIDS Conference opened here on Sunday to UN pleas and activists' clamour for countries not to backtrack in the 29-year war on acquired immune deficiency syndrome.
The six-day forum is expected to draw more than 20,000 scientists, policymakers and grassroots workers to seminars and workshops on scores of issues related to AIDS and the human immunodeficiency virus (HIV).
But the official start at the Vienna congress centre was briefly delayed as several dozen campaigners climbed on stage, waiving placards and demanding funds for fighting AIDS.
In a video message, UN Secretary General Ban Ki-moon said that after arduous effort, "significant progress" was being made against AIDS.
But this could be threatened if countries failed to pursue their commitments, the UN chief said.
"New infections have declined. Access to treatment has expanded. Decades-old travel restrictions are being lifted," he said.
"But too many obstacles remain. Some governments are cutting back on their response to AIDS. This should be a cause for great concern to us all. We must ensure that our recent gains are not reversed."
AIDS funding by the G8 countries and other rich economies fell back slightly last year, to 7.6 billion dollars after 7.7 billion dollars in 2008, as a result of the economic recession, according to an analysis issued in Vienna by the Kaiser Family Foundation and UNAIDS.
In 2009, there was a funding gap of 7.7 billion dollars compared to needs of 23.6 billion dollars.
For 2010, 25 billion dollars has to be mustered but so far, there is a shortfall of 11.3 billion, according to an analysis published this month in the US journal Science.
That contrasts with the Group of Eight's goal, later supported by the UN General Assembly, of securing universal access to AIDS drugs and care by 2010.
Julio Montaner, president of the International AIDS Society which is organising the conference, lashed any leaders who argued funds for AIDS had to be slimmed at a time of belt-tightening.
"Let me remind you that over the last year, the same leaders had absolutely no problem finding the money on a moment's notice to bail out their corporate friends and the greedy Wall Street bankers, yet when it comes to global health, the purse is always empty," he charged.
"A full 110 billion euros [142 billion dollars] appeared from nowhere when the Greek economy faltered earlier this year. But when it comes to universal access, the G8 chose to ignore their commitments before the crisis, and they are poised to continue to do so today."
AIDS has claimed more than 25 million lives since the disease first came to light in 1981. More than 33 million people have HIV, according to UN figures for 2008, which also say new cases of HIV that year rose by around 2.7 million.