Renowned American intellectual accuses the Venezuelan leader of concentrating too much power in his own hands

Hugo Chávez has long considered Noam Chomsky one of his best friends in the west. He has basked in the renowned scholar's praise for Venezuela's socialist revolution and echoed his denunciations of US imperialism.

Venezuela's president, who hasrevealed that he has had surgery in Cuba to remove a cancerous tumour, turned one of Chomsky's books into an overnight bestseller after brandishing it during a UN speech. He hosted Chomsky in Caracas with smiles and pomp. Earlier this year Chávez even suggested Washington make Chomsky the US ambassador to Venezuela.

The president may be about to have second thoughts about that, because his favourite intellectual has now turned his guns on Chávez.

Speaking to the Observer last week, Chomsky has accused the socialist leader of amassing too much power and of making an "assault" on Venezuela's democracy.

"Concentration of executive power, unless it's very temporary and for specific circumstances, such as fighting world war two, is an assault on democracy. You can debate whether [Venezuela's] circumstances require it: internal circumstances and the external threat of attack, that's a legitimate debate. But my own judgment in that debate is that it does not."

Chomsky, a linguistics professor at Massachusetts Institute of Technology, spoke on the eve of publishing an open letter (see below) that accuses Venezuela's authorities of "cruelty" in the case of a jailed judge.

The self-described libertarian socialist says the plight of María Lourdes Afiuni is a "glaring exception" in a time of worldwide cries for freedom. He urges Chávez to release her in "a gesture of clemency" for the sake of justice and human rights.

Chomsky reveals he has lobbied Venezuela's government behind the scenes since late last year after being approached by the Carr centre for human rights policy at Harvard University. Afiuni earned Chávez's ire in December 2009 by freeing Eligio Cedeño, a prominent banker facing corruption charges. Cedeño promptly fled the country.

In a televised broadcast the president, who had taken a close interest in the case, called the judge a criminal and demanded she be jailed for 30 years. "That judge has to pay for what she has done."

Afiuni, 47, a single mother with cancer, spent just over a year in jail, where she was assaulted by other prisoners. In January, authorities softened her confinement to house arrest pending trial for corruption, which she denies.

"Judge Afiuni has suffered enough," states Chomsky's letter. "She has been subject to acts of violence and humiliations to undermine her human dignity. I am convinced that she must be set free."

Amnesty International and the European parliament, among others, have condemned the judge's treatment but the intervention of a scholar considered a friend of the Bolivarian revolution, which is named after the hero of Venezuelan independence, Simón Bolívar, is likely to sting even more.

Speaking from his home in Boston, Chomsky said Chávez, who has been in power for 12 years, appeared to have intimidated the judicial system. "I'm sceptical that [Afiuni] could receive a fair trial. It's striking that, as far as I understand, other judges have not come out in support of her … that suggests an atmosphere of intimidation."

He also faulted Chávez for adopting enabling powers to circumvent the national assembly. "Anywhere in Latin America there is a potential threat of the pathology of caudillismo [authoritarianism] and it has to be guarded against. Whether it's over too far in that direction in Venezuela I'm not sure, but I think perhaps it is. A trend has developed towards the centralisation of power in the executive which I don't think is a healthy development."

Chomsky expressed concern over Chávez's cancer and wished the president a full and prompt recovery.

Chomsky's book Hegemony or Survival: America's Quest for Global Dominance became a publishing sensation after Chávez waved a copy during a UN address in 2006 famous for his denunciation of President George W Bush as a devil.

Its author remains fiercely critical of the US, which he said had tortured Bradley Manning, alleged source of the diplomatic cables exposed by WikiLeaks, and continued to wage a "vicious, unremitting" campaign against Venezuela.

The Chávez government deserved credit for sharply reducing poverty and for its policies of promoting self-governing communities and Latin American unity, Chomsky said. "It's hard to judge how successful they are, but if they are successful they would be seeds of a better world."

Leonardo Vivas, co-ordinator of Latin American initiatives at the Carr Centre, said that Afiuni's case was the most prominent example of the erosion of justice in several Latin American countries. The centre hoped that Caracas would now heed Chomsky.

"He is one of the most important public intellectuals in the US and is respected by the Venezuelan government."

The decision to lobby publicly was taken because quiet diplomacy had limits, said Vivas.

Chávez, who is convalescing in Cuba, has a reputation for lashing back at criticism, raising the risk that the Afiuni initative could backfire.

"That could happen," said Vivas. "But that would mean recognition of the problem."

Chomsky's letter

Judge María Lourdes Afiuni has suffered enough

With this public letter I want to express my open support of the liberty of judge María Lourdes Afiuni, detained in Venezuela since December 2009. In November of last year I was informed of her situation by the Latin American initiative of the Carr Centre for human rights policy at Harvard University. Ever since, I have been directly involved in mediation efforts with the Venezuelan government, with the purpose of releasing her from prison through a gesture of clemency by President Chávez.

Judge Afiuni had my sympathy and solidarity from the very beginning. The way she was detained, the inadequate conditions of her imprisonment, the degrading treatment she suffered in the Instituto Nacional de Orientación Femenina, the dramatic erosion of her health and the cruelty displayed against her, all duly documented, left me greatly worried about her physical and psychological wellbeing, as well as about her personal safety.

Those reasons motivated me in December 2010 to address, jointly with the Carr Centre, a petition for an official pardon from the president in the context of the yearly presidential amnesties.

In January I received with relief the news that Venezuela's attorney general had suggested house arrest for judge Afiuni given her fragile health condition, which ended up with emergency surgery. Being in her house with her family and with adequate medical attention has been without doubt a significant improvement of her situation.

However, judge Afiuni has suffered enough. She has been subject to acts of violence and humiliations to undermine her human dignity. I am convinced that she must be set free, not only due to her physical and psychological health conditions, but in conformance with the human dignity the Bolivarian revolution presents as a goal. In times of worldwide cries for freedom, the detention of María Lourdes Afiuni stands out as a glaring exception that should be remedied quickly, for the sake of justice and human rights generally and for affirming an honourable role for Venezuela in these struggles.

For the above reasons I want Venezuelans to be aware of my total solidarity with judge Afiuni, while I affirm my unwavering commitment with the efforts advanced by the Carr Centre in Harvard University to release her from imprisonment. At the same time, I shall keep high hopes that President Chávez will consider a humanitarian act that will end the judge's detention.

• Read the full transcript of Rory Carroll's interview