The union, representing over 5,000 staff at U.N. headquarters, said it was dismayed at many proposals in Annan's blueprint, especially the call to consider outsourcing a variety of U.N. services from translations to billing.
The disappearance of permanent appointments and a new policy on job mobility without job security implied a "fundamental attack against the international civil service," it said.
The resolution said "in the future, all staff may be at risk" and expressed "a statement of no confidence in the secretary-general and his senior management team."
The staff revolt is just the latest in series of problems the U.N. chief has been force to confront in recent months, including heavy criticism of his management in the scandal surrounding the U.N. oil-for-food program for Iraq.
Annan has also struggled to deal with allegations of sexual abuse by U.N. peacekeepers as well as the fallout from corruption charges linked to how the U.N. awards work contracts.
U.N. spokesman Stephane Dujarric, asked to respond said: "We fully understand that these are unsettling times for the staff but we encourage all staff whether in New York or around the world to read the report fully."
"We look forward to the continuing dialogue we will have with the Staff Union and the discussion that managers will be having with their own staff in each department," he said.
Annan presented his proposal for an overhaul to the U.N. General Assembly on Tuesday and urged its 191 members to invest in management reform so the U.N. can help millions of people around the world facing hunger, disease, violence and terrorism.
The management overhaul won initial support from the United States and the European Union. But Annan and his senior team faced strong objections from U.N. staff, especially about outsourcing and job security, at a raucous and contentious meeting Tuesday afternoon.
The resolution was adopted at an emergency meeting of the Staff Union attended by over 500 U.N. employees. Union leaders said it was approved overwhelmingly, with two people voting "no" and fewer than 10 abstentions.
The proposals are a direct response to last year's investigation into the U.N. oil-for-food program which concluded that shoddy management was partly to blame for widespread corruption. The probe cited weaknesses in oversight, accountability, responsibility and structure.
Since the end of the Cold War in the early 1990s, Annan said, the United Nations has been transformed from an organization that primarily held conferences and meetings to a global body engaged in peacekeeping, humanitarian relief efforts, electoral assistance and human rights monitoring.
The Staff Union said it was "gravely dismayed" at this vision, calling the U.N. "the unique forum where all peoples of the world will continue to come together to discuss global issues, verbally and in writing, in all six official languages of the United Nations."
Annan's proposed shakeup would create a mobile civil service and convert 2,500 short-term peacekeeping positions into a new rapid reaction team whose members could be deployed quickly in urgent peacekeeping and political missions.
It would also allow a one-time staff buyout costing about $100,000 per erson, modernize the U.N.'s information technology systems, and consider relocating, outsourcing and telecommuting a range of U.N. services including translation, editing, printing, publishing, payroll, medical and staff benefits, and information technology support.
Senior U.N. officials said the management reforms could cost about $500 million, but could eventually lead to savings of about the same amount.
The Staff Union's resolution expressed dismay that Annan, on Feb. 22, assured staff representatives that "no strategic decision" had been taken on outsourcing and said the cost of relocating staff would outweigh any savings.
The resolution also expressed dismay that "no one is being held accountable for the failed systems and processes indicated in the report, and their enormous costs for this organization."