The Iranian intelligence ministry has identified a "major terrorist group" linked to Israel and arrested some of its members, state media report.
Bombs, machine-guns and military communications equipment were reportedly seized in operations in Iran's "border and central provinces".
The arrests came after months of complex operations, the ministry said.
In January, Iran accused Israel and the US of the drive-by bombing that killed a nuclear scientist in Tehran.
Israel has not denied any role in the killing of Mostafa Ahmadi-Roshan, the deputy director of the Natanz uranium enrichment plant.
The assassination of the fifth Iranian scientist since 2007 came against a backdrop of growing pressure on Iran over its nuclear programme.
On Tuesday, the intelligence ministry announced it had uncovered "one of large terrorist and subversive networks of the Zionist regime".
"A number of their operatives have been identified in the country and a number of terrorists consisting of criminals and mercenaries collaborating with them have been arrested," said a statement quoted by the official Irna news agency.
"A number of their operatives were arrested while they were getting ready for terrorist operations."
A lengthy intelligence operation had identified "the Zionists' regional headquarters in one of the regional countries and active agents of the headquarters", the statement added.
For security reasons and because of "possible attempts by the Zionist regime to carry out harassing operations", further details about the arrests would not be announced until later, the ministry said.
In March, NBC News quoted two senior US officials as confirming that the assassinations of Iranian scientists had been carried out by members of a dissident group, People's Mujahideen of Iran (PMOI) or Mojahedin-e Khalq, who were financed and trained by Israel's secret service, Mossad.
The officials denied any US involvement in the activities of the PMOI, but a former senior US intelligence official told the New Yorker last week that the operations had benefited from US intelligence.
The New Yorker also quoted a retired general as saying the US Joint Special Operations Command had trained PMOI members at the Department of Energy's Nevada National Security Site in "commo [communications], crypto [cryptography], small-unit tactics and weaponry".
The PMOI, which is designated a terrorist group in the US, has said the allegations that it is working with Mossad are "absolutely false".