"World summits being organised these days are intended to humiliate human beings," Iran's Irna news agency quoted Ahmadinejad as saying.
"These foolish people who are in charge are like stupid, retarded people who brandish their swords whenever they face shortcomings, without realising that the time for this type of thing is over."
Al Jazeera's Alireza Ronaghi, reporting from the Iranian capital, Tehran, said Ahmadinejad's message is not new.
"Ahmadinejad has always been famous for his aggressive tone and he has never held back when it comes to criticising the United States," he said.
"But the timing of his words is important. The Iranian government is trying to prove that there is nothing to worry about when it comes to Iran's nuclear programme, because it is totally peaceful.
"As proof of that, Iran is going to host a nculear disarmament summit ... in Tehran on Saturday."
The US believes that Iran is attempting to build a nuclear weapon in violation of the global nuclear non-proliferation treaty, and it objects to North Korea's nuclear weapons stockpile and exports of nuclear materials and technology.
Rob Reynolds, Al Jazeera's correspondent in Washington, said Iran is not officially on the summit agenda.
But he said the country's disputed nuclear programme would be one of the chief topics of bilateral meetings on the sidelines of the summit, as it was during a meeting between Obama and Jordan's King Abdullah.
"Earlier ... Iran was one of the subjects brought up, [with] President Obama stressing to Abdullah the importance of pressuring Iran to live up to its non-proliferation treaty obligations including via the use of sanctions, if necessary."
"[But] the overall subject matter here is nuclear security and locking down and securing nuclear materials that might be used to create a nuclear bomb or a dirty bomb which could be used by groups such as al-Qaeda."
Global security threat
Obama was expected to use the conference to warn of what he has called "the biggest threat to global security".
"The single biggest threat to US security, both short-term, medium-term and long-term, would be the possibility of a terrorist organization obtaining a nuclear weapon," Obama said at meetings with world leaders on Sunday.
"We know that organisations like al-Qaeda are in the process of trying to secure nuclear weapons or other weapons of mass destruction, and would have no compunction at using them."
Obama set a goal of ensuring all nuclear materials worldwide are secured from theft or diversion within four years.
The White House will seek concrete commitments from world leaders on securing stockpiles of separated plutonium and uranium, to ensure that they cannot be stolen, smuggled or sold.
"I feel very good at this stage in the degree of commitment and sense of urgency that I've seen from the world leaders so far on this issue," Obama said.
"We think we can make enormous progress on this."
The summit will focus primarily on separated plutonium and highly enriched uranium stocks, rather than radiological "dirty" bombs, which the US sees as a less catastrophic threat than nuclear devices.
Checks and balances
Meir Javedanfar, a nuclear analyst, said the summit would also seek to form a coalition among the countries participating so they can "co-operate ... and then take another step further by going to the United Nations."
"I think al-Qaeda could get its hands on nuclear weapons if these checks and balances are not put in place to make sure that nuclear stockpiles around the world are kept in safe places," he told Al Jazeera from Tel Aviv.
US officials hope nations participating in the summit will agree on a series of security steps for their own nuclear material, and help pay to put the stocks of less well-off countries under lock and key.
They also expect some leaders to unveil specific actions, similar to Chile's decision to ship a stock of highly enriched uranium to the US.
Binyamin Netanyahu, the Israeli prime minister, will not attend the conference and is expected to send Dan Meridor, the deputy prime minister, in his place.
Insiders said Netanyahu was worried Turkey and Egypt would use the summit to challenge him over his country's nuclear arsenal, which Israel has never officially acknowledged.
The two countries likely planned to demand that Israel sign up to the 1970 Non-Proliferation Treaty (NPT).