ALMATY, Kazakhstan — A group of well-known writers and poets in Kazakhstan has called on President Qasym-Zhomart Toqaev to release dissident poet Aron Atabek, who has been behind bars since 2007 and is said to be in failing health.

In a letter to the president, published on the Abai.kz website on February 24, the group said the 68-year-old poet, who was sentenced to 18 years in prison after being convicted of helping organize protests that resulted in the death of a police officer, is suffering from heart disease, diabetes, and arthritis.

“If he dies in custody, that will be a shame on all of us!” the letter says, urging the president to release Atabek as soon as possible.

Atabek has maintained his innocence. He rejected a 2012 government pardon offer that would have required him to admit guilt.

In December 2012, after his critical article about then-President Nursultan Nazarbaev and his government was smuggled out of prison and published online, Atabek was transferred to solitary confinement, where he spent two years.

Atabek and his relatives said in 2014 that prison guards had broken his leg, which the authorities denied.

Human rights groups in Kazakhstan say that Atabek has been constantly tortured in prison, with guards intentionally splashing water with high concentrations of chlorine on the floor of his cell to damage his health.

Domestic and international rights organizations have demanded the Kazakh government release Atabek for years.