Two Iranian poets sentenced to long prison terms and floggings have vaulted to international literary prominence over their prosecution, which appears to reflect a tough new crackdown on rights and creative arts in Iran.

On Sunday, the PEN American Center, an advocacy group that promotes free expression worldwide, sent a petition signed by 116 poets and writers to Iran’s supreme leader, Ayatollah Ali Khamenei, beseeching him to grant pardons to the condemned poets, Fatemeh Ekhtesari, 31, and Mehdi Mousavi, 41.

“We are deeply concerned by the inhuman sentences levied against Ms. Ekhtesari and Mr. Mousavi for the simple act of expressing themselves by creating art,” read the letter, which was posted on the group’s website.

Ayatollah Khamenei’s own poetic writings and Iran’s rich history of literature, the letter read, should serve to justify his intervention. “As a poet and scholar of poetry, we appeal to you not to allow this legacy to be clouded by cruel and unwarranted treatment of these two writers,” the letter read.

Ms. Ekhtesari, who has won acclaim for poems about Iranian women, and Mr. Mousavi, known for poems on social issues, join a growing list of writers, artists, lawyers and political dissidents who have offended the conservative anti-Western factions believed to control the judiciary, the police, the military and the intelligence services in Iran.

Despite pledges by President Hassan Rouhani of Iran to soften the repressive atmosphere that prevailed before his election in 2013, rights advocates say that in some ways it has worsened, particularly since Mr. Rouhani’s government reached a nuclear agreement with foreign powers including the United States that stands to ease Iran’s international economic isolation.

Many rights activists view the crackdown as part of a struggle within the government, pitting Mr. Rouhani’s desire to open up the country against hard-liners who view such openness as a threat to their power and the tenets of the 1979 Islamic Revolution.

The same tensions appear to be reflected in the espionage prosecution and conviction of Jason Rezaian, the Washington Post’s Tehran correspondent, a dual citizen of the United States and Iran who has been imprisoned since July 2014, and the recent arrest of Siamak Namazi, an Iranian who has advocated improved relations with the United States.

While Ayatollah Khamenei supports the nuclear agreement, his sympathies also lie with the deeply anti-American suspicions shared by the agreement’s opponents in Iran.

Ms. Ekhtesari and Mr. Mousavi were first arrested in 2013, placed in solitary confinement and interrogated, according to activists following their case. They were released on bail in January 2014, awaiting verdicts on charges that included insulting the sacred in their poems, publishing unauthorized content and spreading anti-state propaganda.

The sentences — 11 and a half years for Ms. Ekhtesari and nine years for Mr. Mousavi, plus 99 lashes for both — were conveyed to their lawyer, Amir Raeesian, on Oct. 10. He has called them baseless and vowed to appeal.

Prosecutors have never made clear which poems were deemed criminal. But a poem from one of Ms. Ekhtesari’s collected works, “A Feminist Discussion Before Boiling the Potatoes,” was the basis for a 2010 protest song by an Iranian musician based in Germany, Shahin Najafi, who was declared to be an apostate in Iran three years ago. Nor has a precise explanation been offered for the flogging punishment, but it may derive from a Swedish poetry festival, where both defendants shook hands with members of the opposite sex.

The PEN petition constitutes a Who’s Who in the Western poetic literary world, laced with best-selling authors and winners of Pulitzers, National Book Awards and other honors. Suzanne Nossel, executive director of the PEN American Center, said that while the group had organized petition drives before, “it was novel to mobilize poets.”

While the Iranian authorities often dispute criticism from American and mainstream human rights groups, Ms. Nossel said, she said she hoped the petition would be an exception in Iran.

“It’s a very literary culture,” she said. “They don’t dismiss poetry.”