At least 15 people were killed on October 26 in an attack on a key Shi'ite Muslim shrine in southern Iran, state media said, with the Islamic State (IS) militant group claiming responsibility for the assault.
State television said the attack carried out by a lone gunman during evening prayers at the Shah Cheragh mausoleum in the city of Shiraz also wounded at least 19 people.
Iranian officials said they had arrested the gunman.
Earlier reports said 13 people were killed and 40 wounded, and that three assailants were involved.
Local judiciary chief Kazem Musavi told state television that "only one terrorist" was involved in this attack.
The assailant "fired indiscriminately on worshipers" gathered at the shrine, local Governor Mohammad-Hadi Imanieh told state television.
Iranian President Ebrahim Raisi vowed "a severe response" while Foreign Minister Hossein Amir-Abdollahian said Tehran will take action against "terrorists and foreign meddlers."
"This crime made the sinister intentions of the promoters of terror and violence in Iran completely clear. There is reliable information that the enemies have drawn up a multilayered project to make Iran insecure," Amir-Abdollahian said in a statement carried by state media.
IS has claimed previous attacks in Iran, including deadly twin attacks in 2017 that targeted parliament and the tomb of the Islamic republic's founder Ayatollah Ruhollah Khomeini.
The attack came on the same day that Iranian security forces clashed with protesters marking 40 days since the death in police custody of Mahsa Amini, a 22-year-old Kurdish woman.
Amini died in police custody in Tehran on September 16 after being arrested three days earlier for "improperly" wearing a mandatory Islamic head scarf, or hijab.
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Iranian Protesters Mark 40 Days Since Death Of Woman Arrested By Morality Police
Her death, which officials blamed on a heart attack, sparked a wave of anti-government protests in cities across the country met by authorities with a harsh crackdown that the Oslo-based group Iran Human Rights says has killed at least 215 people, including 27 children.
Young women and schoolgirls have been at the forefront of the wave of protests, one of the largest Iran's ruling theocracy has been confronted with since the Islamic Revolution of 1979.
On October 26, hundreds of men and women gathered at the Aichi cemetery in Saghez, Amini's hometown in the western province of Kurdistan, despite threats from the security services, which blocked traffic at the entrance into the city, forcing people to walk to the cemetery.
Iran's semiofficial ISNA news agency said about 10,000 people gathered at the cemetery. It also said that the Internet had been cut off in Saghez amid clashes between security forces and mourners.
Hengaw, a Norway-based group that monitors rights violations in Iran's Kurdish regions, also said that security forces teargassed and fired on protesters who massed in the city.
"Security forces have shot tear gas and opened fire on people in Zindan Square, Saghez city," Hengaw tweeted without specifying whether there were any dead or wounded.
The claim could not be independently verified.
Iranian workers and shopkeepers across the country also joined in nationwide protests on October 26.