The Nobel prize-winning author Mario Vargas Llosa has resigned as emeritus president of Pen International after the writers’ freedom of expression group called for the release of two jailed Catalan civil society leaders and claimed Catalans been persecuted “in a way not seen since the Franco dictatorship”.

Jordi Cuixart, the president of Òmnium Cultural, and Jordi Sànchez, former president of the powerful grassroots group the Catalan National Assembly (ANC), have been in pre-trial detention since October 2017.

Both men have been charged with sedition and rebellion over their alleged roles in orchestrating pro-independence protests in September 2017, during which police officers were trapped inside a Barcelona building and their vehicles destroyed.

Jennifer Clement, the president of Pen International, described the charges as “clearly excessive” and said they ought to be dropped.

In a joint statement released on Monday, Pen International and the group’s regional branch, Pen Català, said: “In light of our commitment to defending the right to freedom of expression, we call on the Spanish authorities to drop the charges against Sànchez and Cuixart and to release them immediately.

“Beyond having expressed their ideas peacefully, they have also ensured, from the organisations over which they preside, the free circulation of the ideas of writers in Catalonia.”

The statement was also signed by the Irish writer Colm Tóibín, the Angolan writer José Eduardo Agualusa, and PEN chapters in countries including Mexico and Argentina.

The calls drew a furious response from Vargas Llosa, who was born in Peru but holds Spanish citizenship and has been an outspoken critic of the Catalan independence movement.

In a letter to Clement published in El País on Tuesday, the author said the joint statement had been “full of lies and slanders” against Spain’s democratic government.

Vargas Llosa also described the October 2017 unilateral independence referendum – held unilaterally and in defiance of the Spanish government and constitution – as “an attempted coup” by pro-independence Catalans.

He added: “It’s a shame that an organisation like Pen International, which possesses such clean credentials when it comes to defending human rights and freedom of expression, should buy into the fairytales of the Catalan Pen, which is a militant organ of the Catalan independence movement, which has been rolling out an international campaign to distort the truth and which, judging by your statement, has managed to catch out many Pen groups, among them some Latin American ones, by presenting Spain as a country that crushes freedom of expression and locks up critical and dissident writers.”

Twelve former Catalan leaders are due to stand trial at Spain’s supreme court over the coming weeks charged with rebellion, sedition and misuse of public funds relating to the failed referendum and subsequent unilateral declaration of independence.

