Americans killed hundreds of innocent people in Afghanistan and they continue to attack and kill Iraqi civilians, Khamani, Iran's most powerful figure, told thousands of worshippers at Friday prayers in Tehran.
"They are suppressing the Iraqi people exactly like Saddam Hussein used to," said the senior cleric.
"Who the hell are they to talk about democracy when Americans have occupied Iraq against the nation's will?" Khamani asked.
The remarks of American officials showed they had plans against all the nations of the region, therefore, everyone should be alert, he said.
Khamani said persistent attacks on US forces in Iraq and Afghanistan showed the level of Muslim hatred of Washington.
Today, the Islamic world hates America more than ever because of what happened in Afghanistan and Iraq, he said.
"Who the hell are they to talk about democracy when Americans have occupied Iraq against the nation's will"
Ayat Ali Khamani,
Supreme leader, Iran
The situation in Afghanistan and Iraq clearly shows that America's policy was not successful in the Middle East, Khamani said.
Iran, though it fought an eight-year war with Saddam Hussein's Iraq in the 1980s, is deeply unhappy about US forces in Iraq to its west and Afghanistan to the east. US officials, meanwhile, have stepped up their war of words against the Islamic Republic.
Once close allies, Iran and the US became bitter foes after the 1979 Islamic revolution toppled the US-backed Shah and radical Iranian students stormed the US embassy in Tehran and took dozens of diplomats hostage.
US President George Bush raised the stakes labelling Iran part of an "axis of evil" for alleged backing of terrorism. Washington now accuses Iran of trying to secretly make a nuclear bomb.
US Secretary of State Colin Powell said recently that Iran's "hidebound clerics" had dragged Islam into "the political gutter". Days earlier Bush challenged Iran's rulers to heed what he called the "democratic demands" of the Iranian people.