"Today... this country has joined the countries that produce nuclear fuel on an industrial scale," Ahmadinejad said.
Ali Reza Ronaghi, Al Jazeera's correspondent in Iran, said: "Ahmadinejad never misses a chance of reminding people, and the world, that Iran's nuclear programme - which it claims to be peaceful - has been under a lot of pressure and Iranian scientists have done it without help from the outside world."
Ahmadinejad's speech confirmed an announcement by the head of the country's atomic energy organisation, who also said that Iran had started mass-producing the centrifuges needed for the enrichment of uranium.
"Today, with the start of mass-producing centrifuges and the start of uranium enrichment on an industrial phase, another step was taken for the flourishing of the Islamic republic," Gholam Reza Aghazadeh said.
Ali Larijani, Iran's chief nuclear negotiator, told reporters that 3,000 centrifuges were being used to enrich uranium at the facility.
Experts say Iran could make enough nuclear material for one nuclear bomb a year with 3,000 machines.
Larijani also threatened to withdraw from the nuclear non-proliferation treaty if Tehran were subjected to further international pressure over the issue.
"If they pressure us further we will have no choice but to reconsider our membership of the NPT, as parliament has ruled," he said.
The United Nations Security Council has passed two resolutions since December imposing sanctions on Iran over its nuclear programme.
Sean McCormack, a US State Department spokesman, said in Washington on Monday: "It's another missed opportunity from the Iranians.
"They've had numerous opportunities over the past months to take up the offer that had been extended to them of negotiations so that they can realise their stated nuclear goal of a peaceful nuclear energy programme.
"We in the international system are making the point to them that you are not going to realise any benefits from the current path that you're on. As a matter of fact, you're going to incur a number of costs."Nuclear monitoring
The International Atomic Energy Agency, the UN's nuclear watchdog, has said it has gaps in its knowledge about Iran's plans that must be filled before it can decide whether or not the programme is peaceful.
|Iran insists its nuclear programme is solely |
for energy production [AFP]
The IAEA is pushing Tehran to agree to let it install cameras in the underground section of the Natanz plant to monitor Iran's work, but Iran says such surveillance measures go beyond the basic safeguards it is required to allow.
"I found the speech coming from Ahmadinejad reasonably moderate, many people expected he would adopt a much tougher tone ... and he did not enter into very fine detail of what exactly Iran has achieved," Professor Sadegh Zibakalam, a political analyst from Tehran University, told Al Jazeera.
"I think the gist of Ahmadinejad's speech was defiant against the recent [UN] Security Council sanctions ... he was definitely saying we will not be coerced, we will not be intimated by the United States and the security council."
To mark the nuclear celebration, state television broadcast programming on how enrichment is carried out and detailed how many nuclear plants countries such as the United States and France possessed.
It said atomic progress was a source of "national pride of the highest degree".
Halliburton leaves Iran
Also on Monday, US oil services giant Halliburton, which was formerly led by Dick Cheney, the US vice-president, announced it had completed all contractual commitments in Iran and was no longer working there.
Dave Lesar, Halliburton's chairman and chief executive, said in January 2005 that the company would wind down operations in Iran, which were done through a foreign-owned subsidiary.
Lesar said at the time the services provided were legal, and the company reiterated the point Monday.
"Halliburton's prior business in Iran was clearly permissible under applicable laws and regulations," it said.
A federal grand jury has looked into whether the company or its executives knowingly violated a US ban on trade with Iran.
In a separate statement on Monday, Halliburton said it was co-operating with the federal government's "ongoing investigation" of its business in Iran.