By Agence France-Presse
Since he came to power in 2012, Xi has overseen a crackdown on dissent, with hundreds of lawyers, activists and academics detained and dozens jailed.
His ruling Communist party tolerates no opposition to its rule. Newspapers, websites, and other broadcast and print media are strictly controlled. An army of censors patrols social media and many Western news websites are blocked.
"China and the rest of the world can only be enriched by these opinions and voices," said the letter, organised by the London-based PEN International writers association, which advocates for free speech.
"We therefore urge the Chinese authorities to release the writers, journalists, and activists who are languishing in jail or kept under house arrest for the crime of speaking freely and expressing their opinions," it stated.
Signed by writers including Salman Rushdie, Margaret Atwood and Nobel laureate JM Coetzee, the letter mentioned imprisoned Nobel Peace Laureate Liu Xiaobo, currently serving an 11-year sentence for "subversion", and his wife Liu Xia, who remains under house arrest.
It also referenced, among others, scholar Ilham Tohti, who is serving a life sentence for "separatism" for his criticism of Beijing's policies towards the mostly-Muslim Uighur minority.
Over a dozen members or honorary members of the organisation's China-focused chapter, the Independent Chinese PEN Centre, were currently imprisoned or persecuted, it stated.
"The enforced silence of these friends and colleagues is deafening, and the disappearance of their voices has left a world worse off for this egregious injustice and loss," the letter said.