The cartoonist Ramón Esono Ebalé, arrested in Equatorial Guinea earlier this year, may share the same planet as the writer Neil Gaiman. But according to the latter, they “live in different worlds”.

“I am perfectly free to write whatever I wish, to be as imaginative as I want to be, to create people and places, to challenge the things that I believe need to be challenged, and you are not,” Gaiman declared in a letter to the imprisoned cartoonist. “It is the truth of the worlds that you and I occupy, but it is something that I do not and cannot accept.”

Gaiman is part of a group of international artists and writers, including Ai Weiwei, Kamila Shamsie and Madeleine Thien, who have written letters of hope to imprisoned writers around the world to mark PEN International’s Day of the Imprisoned Writer on Wednesday. According to PEN, Ebalé was arrested in the capital of Equatorial Guinea on 16 September 2017. Security agents initially questioned him about cartoons criticising President Obiang and other government officials, before sending him to prison while investigations were conducted into allegations of money laundering and counterfeiting. He has yet to be formally charged with any offence.

In his letter, Gaiman explained that he was writing “because I need you to know that PEN’s entire community of writers, artists and activists, stands with you. Those who are threatened by your work have tried to silence you, and instead they have amplified your voice. We are with you in your struggle for freedom. And we will continue to stand with you until you are free to carry out your work without fear.”

Writing to the Syrian blogger Razan Zaitouneh, a human rights defender who PEN says was abducted by armed men in 2013, Shamsie echoed Gaiman’s words.

“Those who’ve abducted you might have thought they could shut you up, but your lessons echo so powerfully and will continue to do so,” Shamsie wrote. “We are not giving up – on you, and on everything you stand for. I hope one day, after you’re returned to your family, I’ll have a chance to meet you … I hope the moon is shining strong on your face. And I know that one day, in part because of the work and example of you and your colleagues, the darkness will lift entirely.”

Ai wrote his letter to the poet and artist Zehra Doğan, who is imprisoned in Turkey for “propagandising for a terrorist organisation” in her work as a painter and journalist. Calling for her immediate and unconditional release, Ai wrote: “If a state can sentence an artist, journalist or writer in such a fashion then we are truly living in a dark age where no ideas or creativity can be protected and flourish … Only societies that protect freedom of speech and expression can meet the challenges presented by globalisation and have any chance of a positive future.”

Thien wrote to the blogger Nguyễn Ngọc Như Quỳnh, who has been sentenced to 10 years in prison for “conducting propaganda against the Socialist Republic of Viet Nam”. Thien’s letter said: “I do not know if you know that, around the world, your trial and sentencing caused outrage and heartache, and your case was seen as an egregious example of the Vietnamese government using imprisonment (and consequent separation of families) to punish civil and peaceful discourse. You have shown in your writing and life a commitment to all of us – in Vietnam and outside – in your struggle to protect the basic rights on which we all depend. That your rights were denied – a fair trial, due process, and freedom of expression – is a devastating injustice we now have the responsibility to confront.”

The Mexican poet Homero Aridjis sent his letter to Cesario Padilla, who was convicted of “usurpation” at the National Autonomous University of Honduras in June. Aridjis praised Padilla for “having the courage to raise your voice in a country where criminals act with impunity, and where human rights go unprotected.”

He added: “When I was the international president of PEN, after receiving death threats, I spent a year in Mexico with official bodyguards. I know what it means to live in the shadows because of a justified fear. Having been a student leader, and now being an independent journalist, it is the same for you, now that you are exposed to intimidation and reprisals. I want you to know that your fellow writers, in more than 100 countries around the world, stand in solidarity with you.”