Many western countries fear that the Islamic republic is attempting to arm itself with nuclear weapons, an accusation denied by Tehran, which insists the enrichment is for civilian purposes.
"With great honour, I declare that as of today our dear country has joined the nuclear club of nations and can produce nuclear fuel on an industrial scale," Mr Ahmadinejad said.
It is thought that the Natanz complex has capacity for up to 50,000 of the centrifuges which are used in the enrichment process.
The president was speaking to mark "national nuclear day", the first anniversary of Iran's first successful enrichment of uranium, and, although the celebrations were attended by diplomats from many developing nations, EU members boycotted the events in protest at Iran's refusal of UN demands.
The UN has passed two sets of sanctions against Iran, including a ban on representatives of Mr Ahmedinejad's government from travelling abroad.
The effectiveness of these measures was called into question however when it was revealed that General Mohammad Baqer Zolqadr, deputy interior minister for foreign affairs, took a six-day visit to Russia last week.
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