The two-day conference appeared timed as a counterweight to President Obama's 47-nation summit in Washington last week to discuss nuclear security. Obama did not invite Iran, which the U.S. fears is using a civilian nuclear program as cover to develop a weapons capability. Iran denies that and says its nuclear work is only for peaceful purposes such as power generation.
"The deceptive policy by the sole nuclear offender, which falsely claims to be advocating the non-proliferation of nuclear arms while doing nothing substantive for this cause, will never succeed," Khamenei said.
Iran's conference brought together representatives from 60 countries, including China, Russia, Pakistan, Iraq, Turkey and France, as well as delegates from international bodies and non-governmental organizations, according to Iranian media.
The supreme leader, President Mahmoud Ahmadinejad and several other senior Iranian officials took turns at the podium to warn that America's nuclear policy was endangering the world and encouraging nations to consider withdrawing from the Non-Nuclear Proliferation Treaty.
If the U.S. meant what it said about stopping the spread of nuclear weapons, Israel would not have been able to "turn the occupied land of Palestine into an arsenal with huge stockpiles of nuclear weapons," he said.
At last week's gathering in Washington, world leaders endorsed a call from Obama to secure all nuclear materials around the globe within four years to keep them out of the grasp of terrorists.
Several countries, including Ukraine, Mexico and Canada, declared their intention to give up highly enriched uranium as a step toward making it harder for terrorist groups or criminal gangs to steal or acquire a key ingredient in the making of atomic weapons.
Russia and the U.S. also signed a deal to dispose of tons of weapons-grade plutonium, although that won't start for eight years.
Three sets of U.N. sanctions have failed to pressure Iran to stop its own uranium enrichment work, which it says is only to produce fuel for power stations. The technology is of international concern because it could give Iran a pathway to warhead production.
Iran was angered by Obama's announcement this month of a new U.S. nuclear policy in which he pledged America would not use atomic weapons against nations that do not have them. Iran and North Korea were pointedly excluded from the non-use pledge, and Iranian leaders took that as an implicit threat.
"The insistence of these governments on holding and increasing the destructive powers of these weapons ... serves as a tool of collective intimidation and terror," Khamenei said of the U.S. and other nuclear-armed nations.