The United States and the United Nations on Friday condemned Iranian President Mahmoud Ahmadinejad for branding Israel a "tumor" that needs to be wiped out.
UN Secretary-General Ban Ki-moon Iran's leaders' comments as "offensive and inflammatory."
"The secretary-general is dismayed by the remarks threatening Israel's existence attributed over the last two days to the Supreme Leader and the president of the Islamic Republic of Iran," the UN press office said. "The secretary-general condemns these offensive and inflammatory statements."
"(Ban) believes that all leaders in the region should use their voices at this time to lower, rather than to escalate, tensions," it said in a statement.
"In accordance with the United Nations Charter, all members must refrain from the threat or use of force against the territorial integrity or political independence of any state."
Earlier Friday, US National Security Council spokesman Tommy Vietor told AFP that if Iran was concerned about human rights, "it should stop supporting Syrian President Bashar Assad's brutal assault on the Syrian people."
"We strongly condemn the latest series of offensive and reprehensible comments by senior Iranian officials that are aimed at Israel," AFP quoted Vietor as saying. "The entire international community should condemn this hateful and divisive rhetoric."
"Iran and Syria's blatant disregard for basic human rights is the real insult to humanity," he continued.
Ahmadinejad delivered his speech at a Tehran rally held in honor of Al-Quds (Jerusalem) Day, marked each year on the last Friday of Ramadan in accordance with a tradition established by Ayatollah Ruhollah Khomeini, the late supreme leader of Iran.
Ahmadinejad referred to Israel as a western "tool to dominate the Middle East" and an "insult to all humanity."
"Today, Israel and the Zionist entity are against the preservation of all human rights and human dignity," IRNA quoted Ahmadinejad as saying at the event.
During the same speech, AFP quoted him as saying: "The Zionist regime and the Zionists are a cancerous tumor. Even if one cell of them is left in one inch of (Palestinian) land, in the future this story (of Israel's existence) will repeat."
Later Friday, the Iranian-backed Lebanese group Hezbollah said it possessed what it called precision rockets that could kill "tens of thousands" of Israelis in strikes on Israel.
"I tell the Israelis that you have a number of targets, not a large number ... that can be hit with precision rockets ... which we have," Hezbollah Secretary General Hassan Nasrallah said in a broadcast speech. He said he would not identify the targets and did not say whether the rockets were newly acquired weapons.
Nasrallah said his group could strike a limited number of targets in Israel which if hit would lead to mass casualties - a possible reference to Israeli nuclear facilities, though he said he did not spell out what he meant.
Israel, the only Middle East country outside the nuclear Non-Proliferation Treaty (NPT), has never confirmed or denied having nuclear weapons.
"Hitting these targets with a small number of rockets will turn ... the lives of hundreds of thousands of Zionists to real hell, and we can talk about tens of thousands of dead," said Nasrallah.
Nasrallah said Israel was still debating whether to attack Iran because "Iran was strong and brave."
"We all know that the Islamic republic's response will be very great and thunderous if it is targeted by Israel," he said.
Protesters held massive rallies in cities across Iran on Friday, marking the anti-Zionist event which calls for the liberation of Palestine.
Demonstrators waved Palestinian flags and held signs bearing slogans such as "Down with the US" and "Death to Israel" at Friday's rallies.
Iran's Fars News Agency reported that millions of people were expected to take part in the events in over 550 cities throughout the country. The report added that the demonstration was not only against Israel and the US, but the silence of the international community at "the crimes committed in the occupied lands."
Al-Quds Day rallies were also scheduled to take place in other countries throughout the Muslim world on Friday.
The Iranian show of support for the Palestinian cause comes amid tension between Israel and the Islamic Republic regarding the Iranian nuclear program. Israel has demanded that the Iranians halt the development of their nuclear technology, threatening a military strike if its demands were not met. Iran has responded by asserting that its nuclear program is exclusively for non-military purposes and decrying Israel's constant threats as "psychological warfare."
Ayatollah Khomeini established Al-Quds day in 1979 following the Islamic Revolution and wanted to "export" the event to the rest of the Muslim world. Khomeini's successor, Supreme Leader Khamenei, reaffirmed his commitment to the Palestinian cause Wednesday, stating that he is confident that "the fake Zionist (regime) will disappear from the landscape of geography,” Iran's Mehr News Agency reported
Herb Keinon and Joanna Paraszczuk contributed to this report