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."