Centrifuges used for enrichment were disconnected at the Nantaz plant, according to TV.
The move is part of a deal reached with the US, Russia, China and European powers last November.
The UN nuclear watchdog, the IAEA, will now confirm whether Tehran is implementing its side of the agreement.
This should pave the way for partial suspension of EU and US sanctions, allowing Iran to restart petrochemical exports and trade in gold, worth billions of dollars.
“The IAEA inspectors in the Natanz plant are disconnecting cascades,” the head of Iran’s Atomic Energy Organization, Ali Akbar Salehi, said.
“The sanctions iceberg against Iran is melting.”
The West accuses Teheran of seeking nuclear weapons, but it denies the claim, saying its program is solely for peaceful purposes.
Verification by IAEA inspectors that the terms of the deal are being implemented by Teheran is expected to happen by the end of Monday.
Within an hour or two restrictions on Iran’s trade would then be lifted.
On Sunday, Iranian Foreign Minister Mohammad Javad Zarif said he hoped for “positive results for the country, as well as regional and global peace and security”.
Under the terms of the agreement, reached with the five permanent members of the UN Security Council plus Germany, Iran has agreed to halt enrichment of uranium above 5% purity.
It will “neutralize” its stockpile of near-20%-enriched uranium.
In return, the world powers agreed to suspend certain sanctions on trade in gold and precious metals, Iran’s automotive sector, and its petrochemical exports.
Officials said as a result of the deal:
In return, US President Barack Obama has said the US and the other five powers over the next six months will begin to implement “modest relief” so long as Iran fulfill its obligations.
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