Iranian police say they detained about 21,000 people during the 12-day war with Israel in June. This number far exceeds previous official estimates. Police spokesman Saeed Montazerolmehdi said on 12 August that public tip-offs played a significant role in the arrests.
It is unclear how many people remain in custody.
Among those detained, 2,774 were foreign nationals accused of spying. Authorities say evidence on their phones indicated suspicious activity, including images of military positions, key sites and transmitted location data. A further 261 people were suspected of spying and 172 arrested for unauthorised filming.
The announcement stands in stark contrast to comments by Gholam Judiciary Chairman Hossein Mohseni-Ejei, who in late July reported some two thousand arrests during and after the conflict, saying many of those detained were released after investigations failed to confirm suspicions of espionage or collaboration with Israel.
Human rights groups have expressed alarm at the scale of the crackdown.
The US-based human rights activist news agency HRANA has documented hundreds of arrests during the conflict, often targeting ethnic minorities and activists. Amnesty International has condemned the post-war rise in executions in Iran, warning of torture, unfair trials and killings.
Since late June, Iran has executed at least seven people accused of spying for Israel, including nuclear scientist Rouzbeh Vadi on 6 August.
Vadi, who was arrested more than a year ago, was accused of providing sensitive intelligence information that facilitated Israeli operations.
In June, parliament passed a bill to sharply increase penalties for spying and collaborating with hostile foreign governments, particularly Israel. Although the Board of Trustees rejected the bill, a spokesman for the Constitutional Oversight Body said that with amendments it could still become law.