Iran has started halting uranium enrichment to 20 percent and has begun down-blending uranium enriched to higher levels, the state-run Fars news agency reports.
The UN nuclear agency has confirmed that Iran had halted the most sensitive uranium enrichment work.
Monday's unplugging of the equipment is part of an interim deal between Tehran and world powers to power down key nuclear equipment for six months in return for an easing of crippling sanctions imposed by the West.
As part of the nuclear freeze, Iran has to halt the enrichment of uranium to medium levels, close to weapons-grade, and to begin diluting half of its stockpile of this material.
After the International Atomic Energy Agency (IAEA) confirmed the work had begun, EU foreign ministers adopted legislation loosening sanctions on items such as auto parts and gold, to be followed later in the day by a similar move in Washington.
Over the next six months Iran will also not install or switch on new nuclear machines and will grant the IAEA more access, including daily visits to the Fordo and Natanz enrichment facilities.
The total sanctions relief, staggered over the six months, is worth about $6-7bn, including $4.2bn in frozen overseas assets, with the first $550m installment due on February 1.
Iran and the P5+1, made up of the US, China, Russia, Britain, France and Germany, will soon begin talks on a long-term comprehensive accord.
Threatening to put an end to the deal, however, is a push by US politicians, including some from President Barack Obama's own party, to impose new sanctions on Iran, even though this would contravene the November deal.
The pro-sanctions camp believe it has at least 59 votes in the Senate and a healthy majority in the House of Representatives and could be approaching the two-thirds majority needed to override the veto Obama has promised.