US Secretary of State Mike Pompeo accused Iran of "an outrageous and unwarranted act of intimidation," while the European Union voiced "deep concern".
Iran's Atomic Energy Organisation spokesman Behrouz Kamalvandi said that a check at the entrance gate to the Natanz uranium enrichment plant "triggered the alarm multiple times, showing (the inspector) was either contaminated with certain materials or had them on her".
He did not specify what the materials were or whether they had actually been found in her possession.
Kamalvandi said that Iran's report on the incident to the International Atomic Energy Agency had convinced everyone but "the US, the Zionist regime and some Persian Gulf countries".
"We've announced that if needed we will even present the footage of this," he told a news conference, noting that Iran's "bitter experiences" of nuclear sabotage had led to the strict system of checks.
Iran has accused its arch foes Israel and the United States of mounting a long campaign of sabotage involving the assassination of Iranian engineers and cyberattacks on key facilities.
The IAEA said Thursday that the inspector was briefly prevented from leaving the country, adding that her treatment was "not acceptable."
Iran's ambassador to the agency, Kazem Gharib Abadi, denied the inspector was ever detained, saying she was allowed to leave even though an investigation was still ongoing.
Under a 2015 deal between Iran and major powers that has been undermined by Washington's withdrawal last year, its nuclear facilities are subject to continuous monitoring by the IAEA.