A showpiece summit hosted by Iran stumbled as soon as it opened on Thursday when the head of the UN pressed Tehran on its nuclear stand, and Egypt's new leader publicly sided with Syria's opposition.
The double challenge, before the leaders and delegates of the 120-member Non-Aligned Movement, upset Iran's plans to portray the two-day summit as a diplomatic triumph over Western efforts to isolate it.
Supreme leader Ayatollah Ali Khamenei opened the e vent with a speech blasting the United States as a hegemonic meddler and Israel as a regime of "Zionist wolves."
He also stated that his country "is never seeking nuclear weapons" and accused the UN Security Council, under U.S. influence, of exerting an "overt dictatorship" over the world.
UN chief Ban Ki-moon, who looked irritated at Mr. Khamenei's remarks, shot back that Iran should boost global confidence in its nuclear activities by "fully complying with the relevant (UN) Security Council resolutions and thoroughly cooperating with the IAEA," the UN's nuclear watchdog.
He warned about the current state of bellicose rhetoric coming from Israel and Iran, saying "a war of words can quickly spiral into a war of violence."
Egypt's new President Mohamed Morsi – making the first visit to Iran by an Egyptian head of state since the 1979 Islamic revolution – in turn embarrassed his hosts by voicing support for the opposition in Syria, which is fighting the Damascus regime unwaveringly backed by Iran.
"The revolution in Egypt is the cornerstone for the Arab Spring, which started days after Tunisia and then it was followed by Libya and Yemen and now the revolution in Syria against its oppressive regime," Mr. Morsi said.
That contradicted the line put out by Damascus and Tehran, which assert that the Syrian uprising is a "terrorist" plot masterminded by the United States and regional countries.
Mr. Morsi's address prompted a walkout by the Syrian government delegation and drew a sharp response from Syria's Foreign Minister Walid Muallem, who accused the Egyptian leader of inciting further bloodshed in Syria.
Iran's state media failed to mention the contentious parts of Ban and Morsi's speeches in their coverage of the summit.
The summit to-and-fro over Iran's nuclear ambitions had its roots in an unusually frank meeting Mr. Ban held with Mr. Khamenei and Mr. Ahmadinejad after arriving on Wednesday.
Mr. Ban told them Iran needed to provide "concrete" steps to ease the international showdown which has raised the spectre of airstrikes on nuclear facilities, threatened by both Israel and the United States.
Tensions have been raised by the International Atomic Energy Agency unveiling a new Iran "task force" to scrutinise Tehran's nuclear programme and its compliance with UN resolutions.
Additionally, the latest IAEA report on Iran's nuclear progress was expected to be released this week – possibly even during the Tehran summit.
The report is said to highlight expanded enrichment in Iran and suspicions concerning an off-limits military base in Parchin, outside Tehran, where warhead design experiments might have taken place.
Mr. Ban, whose presence at the summit had been criticized by the United States and Israel, also took Iran's leaders to task for recent comments calling Israel a "cancerous tumour" that should be cut out of the Middle East. He urged both Iran and Israel to cool the bellicose language.
"I strongly reject any threat by any (UN) member state to destroy another, or outrageous comments to deny historical facts such as the Holocaust," Mr. Ban said in his summit speech.
"Claiming another UN member state does not have the right to exist or describe it in racist terms is not only utterly wrong but undermines the very principles we have all pledged to uphold," he said.
"I urge all the parties to stop provocative and inflammatory threats. A war of words can quickly spiral into war of violence. Bluster can so easily become bloodshed. Now is the time for all the leaders to use their voices to lower, not raise, tensions," he said.
A total of 29 heads of state or government are attending the Tehran summit, including those of Afghanistan, India, Iraq, Lebanon, Pakistan, the Palestinian Authority, Sudan, Qatar and Zimbabwe. North Korea was represented by its ceremonial head of state, parliamentary president Kim Yong-Nam, rather than the country's leader Kim Jong-Un.