Crowds chanted "revenge, revenge" during the funerals in the city of Kerman, the scene of Wednesday's explosions, the bloodiest such attacks in Iran since the 1979 Islamic Revolution.
Nearly 100 people were killed in Wednesday's assaults at a memorial for top commander Qassem Soleimani, who was assassinated in Iraq in 2020 by a U.S. drone.
The explosions came as regional tensions soared and Israel's war against Hamas in Gaza neared the three-month mark.
Iran's interior minister told state TV a number of suspects had been arrested.
"Our country's capable intelligence agencies have found very good clues regarding elements involved in the terrorist explosions in Kerman and a section of those who had a role in this incident have been arrested," Ahmad Vahidi said without elaborating.
Islamic State said on Thursday two of its members had detonated explosive belts in the crowd that had gathered for Soleimani's memorial in the southeastern city.
"We will find you wherever you are," Revolutionary Guard Corps commander Major-General Hossein Salami said at the funeral in Kerman's Imam Ali religious centre.
"Our enemies can see Iran's power and the whole world knows its strength and capabilities," President Ebrahim Raisi said in a televised address. "Our forces will decide on the place and time to take action".
In 2022, Islamic State claimed responsibility for a deadly attack on a Shi'ite shrine in Iran that killed 15 people, while earlier attacks claimed by Islamic State include twin bombings in 2017 that targeted Iran's parliament and the tomb of the Islamic Republic's founder, Ayatollah Ruhollah Khomeini.
(dubai.newsroom@thomsonreuters.com; Editing by Ros Russell and Andrew Heavens)
