Zimbabwe's former Vice-President Emmerson Mnangagwa, who was sacked on Monday, has fled to South Africa amid death threats, his allies say.
President Robert Mugabe, 93, accuses his former deputy of plotting to take power from him.
"He went to apostolic church prophets to find out when Mugabe would die. But he was told he would die first," Mr Mugabe told supporters on Wednesday.
First Lady Grace Mugabe is now the favourite to succeed her husband.
Mrs Mugabe had been pushing for the removal of the vice-president, referring to him as a snake that "must be hit on the head."
Although Mr Mnangagwa has not commented in person, the angry, five-page statement attributed to him says he had been loyal to President Mugabe.
He reportedly said that he was being "vilified" by people who draw their power "not from the people or party but from two individuals in the form of the first family".
The statement said the former vice-president said he planned to return to lead Zimbabwe and accused President Mugabe and his wife of privatising and capturing the party.
It also criticised Zanu-PF officials for allowing the party to be hijacked, saying that it was "not personal property".
"We must reject this insane and 'idiotic' habit of expelling and suspending members of the party merely because we differ in opinion or have brighter and more progressive ideas of improving the lives of our people," the statement added.
Possible treason charge
Shingai Nyoka, BBC News, Harare
Given that this statement has been issued by a third party, Emmerson Mnangagwa could disown it in future.
Last year, the same group of war veterans who are speaking on his behalf reportedly issued an unsigned communiqué criticising "genocidal" President Mugabe and labelling him a dictator.
They disavowed the statement when they were arrested.
If this five-page letter is from Mr Mnangagwa, he would probably be arrested on his return to Zimbabwe and charged with treason.
Although the letter is not specific about the actions to be taken so that he can lead Zimbabwe, ordinary Zimbabweans have been charged for trying to overthrow the government for merely posting critical tweets.
Mr Mnangagwa, a former intelligence chief, had been a leading candidate to succeed President Mugabe.
He had worked closely with Mr Mugabe for more than 40 years, serving as his special assistant during the liberation war in 1977.
His nickname in the party was the "crocodile", so-called due to his shrewdness. As yet, he has not commented on his dismissal.