The letter was submitted to Iranian opposition activist Mohammed Nourizad, who posted it on his website this week.
The former general wrote that he had been dismissed for refusing to participate in criminal activities on behalf of the Iranian regime, adding that Khamenei was complicit in a pattern of brutal suppression of dissent.
I'm writing this letter to you to tell our people that there are still many generals and members of staff within the Revolutionary Guards who are opposed to these crimes and are waiting to join the people, said the letter.
Nourizad told the Guardian that he was sure of the letter's legitimacy since the former general had handed the message to him in person. The activist added that he had received similar letters from other Revolutionary Guard members in the past.
The letter reflects a clear current of dissent within Iran, where an opposition movement bubbles just below the surface despite constant suppression and regular censorship.
Iran, which faces severe diplomatic isolation and economic sanctions due to its alleged pursuit of nuclear weapons technology, is facing challenges from all sides. The crisis has deepened under President Mahmoud Ahmadinejad and Supreme Leader Khamenei, but both seem determined to hold onto power at all costs.
Opposition to the Iranian government has been growing slowly for three decades. As the former Revolutionary Guard member asks in his letter, How can a supreme leader with blood on his hands be close to God?
Ever since a popular 1979 revolution, which overthrew an increasingly secular monarchy in favor of an Islamist democratic theocracy, more and more power has been consolidated into the hands of the ayatollah. The change was initially celebrated but has since given way to a growing dissatisfaction, especially among Iranians who were too young to have taken part in the 1979 movement.
Today, those youths make up over 60 percent of the population.
The Revolutionary Guard was formed just after the revolution. The troops, which number over 100,000, are meant to protect and enforce Iran's Islamist system. Iran's military is a separate entity, concerned more with national defense and other security issues.
But these roles have overlapped, and the Revolutionary Guard -- which is loyal to Khamenei -- has taken a greater role in matters both within and beyond Iran's borders.
Over the years, the regime became more and more repressive, and growing unrest culminated in a failed revolution three years ago: the Green Movement. This uprising followed the 2009 presidential elections, which saw the incumbent Ahmadinejad win a second term. Demonstrators claimed that the election was rigged and demanded a revote.
Forces loyal to Ahmadinejad and Khamenei suppressed the uprising, and the Revolutionary Guard played a prominent role. They supervised the Basij Resistance Force, a volunteer militia that was instrumental in the clampdown. Over 5,000 demonstrators were arrested, and dozens lost their lives.
The protests fizzled after several months due to intimidation and media censorship, leaving the middle-class urban youths who formed the backbone of the opposition jaded but unable to openly express their dissatisfaction.
Ahmadinejad's mandate has been upheld until the next presidential elections, which take place in 2013. Today, the biggest threat to his power is not urban youths, but the Supreme Leader himself. Khamenei and Ahmadinejad were once closely allied, but they have drifted apart in recent months.
This was was made clear in a recent public rift, when the president tried to sack an intelligence minister supported by Khamenei. The cleric's cronies berated Ahmadinejad, charging him with overstepping the bounds of his power. Squabbles continued, and the president still faces the threat of being politically sidelined, or even fired.
In that context, this week's letter from the former Revolutionary Guard member adds yet another layer to the story of hidden conflicts within Iran's elite political circles.
According to him, the 2009 suppression of Green Movement was a turning point for many disillusioned Iranians. It motivated a great many Revolutionary Guards to question their devotion to the Iranian regime.
They shut our mouth for years by saying that the leader wanted this or that, he wrote. But we could no more keep it shut after the post-election bloodshed.
Just last year, for instance, during the first bloom of the Arab Spring, a group of senior officers in the Revolutionary Guard wrote their own letter to leadership in which they asserted that they would not open fire on civilians who engaged in peaceful protests.
We promise our people that we will not shoot nor beat our brothers who are seeking to express legitimate protest against the policies and conduct of their leader, said that missive, according to the Telegraph. Clearly, Nourizad's mystery general was not the first guard member to take issue with the regime's tactics -- and he probably won't be the last.
Meanwhile, Iranian youths continue to feel the pinch of international sanctions against the regime, a saga that plays out against the backdrop of protests and new governments sprouting up all across the Middle East. The question is whether Khamenei could rely on the Revolutionary Guard, as he did in 2009, to keep a resistance from growing into a powerful national movement. If not, restive Iranians may soon feel emboldened to make a new attempt at revolution.