Ahmadi was arrested in Mazandaran, northern Iran, three days after she published on YouTube her unveiled performance in a caravanserai, her lawyer Milad Panahipour told Emtedad news website.
"We have no information about the charges against Parastoo Ahmadi, the arresting authority, or her place of detention," Panahipour said.
Two members of her band, Ehsan Beiraghdar and Soheil Faghih-Nassiri, were also arrested on Saturday, according to the lawyer. All three were released by Sunday morning.
On December 12, the Judiciary of the Islamic Republic issued a statement saying that the concert was held "without legal authorization and adherence to Sharia principles" and that "appropriate action" will be taken against the singer and production team.
The Caravanserai Concert, held on Wednesday night, quickly became one of the most discussed topics on Iranian social media.
Despite YouTube being restricted in Iran, the performance has attracted over 1.5 million views over the past three days, with clips widely shared online.
Ahmadi, in her introduction, described the concert as hypothetical, saying, “I am Parastoo; a girl who wants to sing for the people she loves... this is a right I could not forsake.”