One of Robert Mugabe’s oldest and closest allies has launched a scathing attack on his presidency, accusing him of trying to turn Zimbabwe into a monarchy by lining up his wife to succeed him.

Rugare Gumbo, who has known Mugabe for 50 years, was this week expelled from the ruling Zanu-PF party amid bitter factional infighting. On Thursday the president told a party congress there was a “treacherous cabal” bent on removing him from power.

But Gumbo rejected the charge and accused the 90-year-old of authoritarianism as he seeks to appoint his controversial wife, Grace Mugabe, as his deputy and heir apparent.

“I feel betrayed,” Gumbo said in his first interview since his expulsion. “After serving my country for so many years and being involved in the liberation for such a long time, I feel really let down because we could have worked together until his demise.”

For years, Gumbo served as Zanu-PF’s information and publicity secretary, defending the president and party against allegations of repression, human rights abuses and election rigging. Now, cast into the political wilderness, he speaks more freely.

“The way things are happening, it’s hard not to conclude that there is amount of dictatorship, there is an amount of tyranny, there is an amount of authoritarianism,” he told the Guardian in the capital, Harare.

“If you’re going to appoint your vice-president, that smacks of authoritarianism in this age when people should be elected. To me, it doesn’t make sense.”

Gumbo claimed that he, vice-president Joice Mujuru and other senior figures are being purged because they objected to Mugabe’s plan to name himself president for life and Grace – dubbed “DisGrace” and “First Shopper” by critics – as his deputy instead of holding internal elections.

“Mugabe said he wants to rule for ever. We said no, it’s not right, it’s not democratic. It’s not right that you hold a congress with unelected people. For some of us, that’s unacceptable.”

The president intended to build a dynasty, Gumbo added, starting with the first lady. “All the signs seem to be leading to that. These are appointments so there’s no ’accepting’ it. I don’t like the idea of monarchy, as it were. I’ve no problem with women competing to be president, but certainly I don’t like the way they’re trying to impose her. We want refined leaders, people who have experience. I don’t think she has the experience.”

Under fire … vice-president Joice Mujuru. Photograph: Tsvangirayi Mukwazhi/AP

Grace Mugabe has publicly denounced Mujuru, seen as a relative moderate, and the state-controlled Herald newspaper has launched fierce attacks on Mujuru’s character. In recent days, Mugabe has accused the vice-president of plotting to assassinate him and even asking witch doctors to cast a magic spell using water insects named Mugabe and Mujuru.

Gumbo, 74, responded: “The rubbish they’re talking about, trying to assassinate him … They used the word ’assassinate’ to frighten people. It pains some of us because, to be honest, we never ever organised his ouster. Joice Mujuru was so loyal to Mugabe that you could never say anything against him in her presence. To talk about a coup is unbelievable.”

Gumbo first met Mugabe in prison in 1964 during the liberation struggle against white minority rule in what was then Rhodesia. He is the only surviving member of Zanu’s war council and pushed for Mugabe to become leader of the movement when others had doubts. “I’ve lived to regret that decision,” he said.

When the pair fell out in Mozambique, Gumbo was held captive in a pit that was six metres deep, four metres long and two metres wide, he recalled. “It was terrible.”

Eventually, he got back in favour and joined the government in 2000. He has regularly attended meetings of Zanu-PF’s decision-making politburo under Mugabe for the past decade.

Looking back on the life and career of Africa’s oldest president, he reflected: “He was really bright, alert, articulate, committed to the nationalist cause. He was really quite radical. But his personality changed of late. He’s no longer himself. He was young and vibrant; now he’s a tired old man.”

Neither Gumbo nor Mujuru was present at the Zanu-PF congress, held in a cavernous tent on a field hastily named “Robert Mugabe Square” with a service road called “Dr Grace Mugabe Way”.

Mugabe threatened to turn the law on 59-year-old Mujuru, telling the crowd: “As we thought we were working together, they were doing their own thing, a cabal parallel to the party, planning their own future, planning how to change the leadership, planning about kicking the president out of power.”

Speaking in Shona, he described Mujuru as a “thief” and “crook” and accused her of “gross corruption” in several gold and diamond-mining deals. “Those who are or were involved in corrupt activities, you are going to be prosecuted wherever we have enough evidence,” he warned.

Wearing a yellow baseball cap and jacket bearing his face, Mugabe appeared drained at the end of his speech. But Grace, wearing matching colours, smiled as she led 12,000 delegates singing and dancing. Parliamentary speaker Jacob Mudenda gave a vote of thanks, referring to Grace as “mother of the nation and mother of revelations”.

Mujuru took part in the 1970s guerrilla war, and was known as Teurai Ropa (Spill Blood). She married Solomon Mujuru, a former army chief who died in a fire at his farm in 2011.

The congress ends on Saturday with the appointment of new leadership. Grace is set to become head of the Zanu-PF women’s league, giving her a seat on the politburo. Despite his advanced years and rumours of cancer, Mugabe is running unchallenged as Zanu-PF’s candidate for the 2018 general election. He will then be 94 years old.