Asha-Rose Migiro criticised "violence and intimidation" in Zimbabwe's run-off poll
The UN has urged African leaders at a key summit in Egypt to try to negotiate a solution to the crisis in Zimbabwe.
UN Deputy Secretary General Asha-Rose Migiro said this was the "moment of truth" for the African Union leaders.
President Robert Mugabe is attending the meeting. He was sworn in on Sunday after a victory that observers said had been undermined by pre-poll violence.
South Africa has now urged Mr Mugabe to hold talks with the opposition towards forming a transitional government.
Mr Mugabe claimed a landslide victory as the sole candidate after the leader of the opposition Movement for Democratic Change (MDC), Morgan Tsvangirai, withdrew.
The South African presidency said it would consider reports from election observers together with other members of the Southern African Development Community (Sadc) before adopting a unified position regarding the result.
Ms Migiro told the leaders of the 53-nation AU: "This is a moment of truth for regional leaders... the secretary-general urges your excellencies to mobilise support for a negotiated solution."
She added: "This is the single greatest challenge to regional stability in southern Africa."
Ms Migiro again expressed UN regret that the election had been allowed to go ahead despite the violence.
The vote fell short of the African Union's standards of democratic elections
In his welcoming speech, host President Hosni Mubarak said bolstering peace and security was "essential for resolving disputes and conflicts in the continent".
The AU has a rule not to accept leaders who have not been democratically elected - but observers say it is unlikely to take such strong action against Mr Mugabe so quickly.
A draft resolution written by African foreign ministers during talks ahead of the summit did not criticise the elections or Mr Mugabe, but condemned violence in general terms and called for dialogue.
President Jakaya Kikwete of Tanzania, the African Union chairman, pointedly congratulated the people of Zimbabwe over the election, but not Mr Mugabe himself.
He said he "commiserated with them for their suffering" and said there were "even more challenges" ahead.
Africa's longest serving leader, Gabon President Omar Bongo, has given the strongest suggestion of recognising Mr Mugabe as president, saying "he was elected, he took an oath, and he is here with us, so he is president".
Independent observers have criticised the poll.
The AU's own monitors said on Monday: "The vote fell short of the African Union's standards of democratic elections."
ZIMBABWE AND ITS NEIGHBOURS
Zimbabwe's opposition wants neighbouring countries to persuade
Robert Mugabe to step down. So how are relations changing?
South Africa's leader Thabo Mbeki remains the key mediator. He has not
criticised Mr Mugabe, despite pressure from the ruling ANC.
Zambian President Levy Mwanawasa called Zimbabwe a "regional
embarrassment", before suffering a stroke on 29 June.
Angola's President Jose Eduardo dos Santos is one of Robert Mugabe's closest allies. He has urged Mr Mugabe to end the violence.
Botswana said Zimbabwe's 27 June run-off vote was so flawed by violence
that it could not be considered legitimate.
Namibia is an ally of Robert Mugabe. It wants to re-distribute white-owned farms to black villagers. It has not criticised the violence.
Mozambique has hosted some white farmers forced out of Zimbabwe when their land was seized. It is seen as sympathetic to the opposition.
Tanzania's ruling party has a history of backing Robert Mugabe's
Zanu-PF party. Its foreign minister has condemned the violence.
DR Congo's President Joseph Kabila is an ally of Robert Mugabe who
sent troops to help his father, Laurent Kabila, fight rebels.
Malawi is seen as neutral. But 3m people from Malawi are in Zimbabwe
and many were badly hit by the farm invasions.