A former general of Iran's powerful Revolutionary Guards has accused the supreme leader, Ayatollah Ali Khamenei, of having blood on his hands over the brutal crackdown on the opposition, and described government claims that its nuclear programme is entirely peaceful as a "sheer lie".

In a letter to prominent opposition activist Mohammad Nourizad (website in Farsi), the former officer gives a rare glimpse of political dissent within the ranks of the elite force in charge of the nuclear programme and Khamenei's personal security.

Identified only by his initials, the general says that he and a number of his colleagues were threatened with execution for disloyalty and then – after a series of secret courts-martial – dismissed "because we refused to participate in the betrayals and the crimes committed by our seniors".

"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," the letter reads.

Speaking to the Guardian by telephone from Tehran, Nourizad – who published the general's letter on his website – said he was convinced of its authenticity because it was handed to him in person by the former general.

"This is one of the many such letters written by senior figures within the Sepah [Revolutionary Guards] that I have received. I have refrained from publishing many of them because I was worried they might pose security problems," said Nourizad.

Iran's Revolutionary Guards are an elite force separate from the Iranian army and under the direct command of the country's supreme leader. The corps was founded to protect Iran's revolutionary values, but has become increasingly involved in politics and business.

In the aftermath of the fiercely disputed 2009 elections, several dozen Revolutionary Guards generals, as well as senior figures in the informal voluntary Basij militia, were replaced for refusing to use violence against unarmed protesters.

According to the general, the order to open fire on the protesters came from the top. "[In 2009,] the leader [Khamenei] asked Rahim Safavi [a former chief commander of the guards] whether he would be prepared to run over people with tanks if they took to streets to revolt. He said yes and the leader gave him the order," he writes.

"They shut our mouth for years by saying that the leader wanted this or that … But we could no more keep it shut after the post-election bloodshed. This was the point many spoke out or simply refused to comply."

According to the general, Khamenei was also personally involved in the restrictions imposed on the opposition leader Mir Hossein Mousavi, who is still under house arrest with little access to the outside world. "How can a supreme leader with blood on his hands be close to God?," he asks.

The general also accuses Khamenei of lying about Iran's nuclear programme, which is now subject to an international dispute, the general casts doubt on what the regime officials claim to be "peaceful" activities, describing them as the country's "nuclear gamble".

"The inspectors of the International Atomic Energy Agency are fooling themselves if they believe that the nuclear facilities on and under the ground are only for peaceful purposes," he writes. "The leader said [in a fatwa] that Iran has only peaceful intentions with its nuclear activities. This is a sheer lie."

"We undertook this nuclear gamble with the leader's knowledge – that's why we are paying billions of dollars into Chinese and Russian bank accounts so that they support us in international negotiations and we could find a way out of this stalemate." Opposition activists often accuse the Iranian government of securing political support from China and Russia with preferential oil contracts and business opportunities.

Amid strict economic sanctions and growing tensions in the Gulf, Iran has periodically threatened to block the strait of Hormuz, a passageway through which 20% of the world's oil supply passes. But the general said that in the event of any hostilities, Iran's forces in the Gulf would not be able to resist for more than a day.

The general accuses the Revolutionary Guards of corruption and involvement in smuggling and the illegal transfer of foreign currency. "Is it not ridiculous? The presidential office established a unit to combat smuggling but at the same time, senior guards generals were doing exactly the same thing in the southern islands of Qeshm, Hormuz and Abu Musa," he writes. "We tolerated the guards' economic activities and involvement in smuggling with the excuse of raising funds for its revolutionary ideas."