Iran already has about 3,000 centrifuges operating in Natanz, and the new announcement is seen as a show of defiance of international demands to halt a nuclear program the United States and its allies say is aimed at building nuclear weapons.
"The president announced the start of the phase of installing 6,000 new centrifuges in Natanz," state television reported.
Centrifuges are machines that can enrich uranium to a low level to produce nuclear fuel or a high level for use in a weapon. Iran insists its nuclear program is peaceful and solely focused on the production of energy.
Ahmadinejad made Tuesday's announcement as he toured the Natanz facility in central Iran. State television also quoted Ahmadinejad as saying that "other activities have been carried out" in Natanz that he would announce later Tuesday.
The president's trip was scheduled to coincide with Iran's National Day of Nuclear Technology, marking the second anniversary of Iran's first enrichment of uranium.
Ahmadinejad is widely expected to confirm for the first time that Iran has installed hundreds of more sophisticated centrifuges that can enrich uranium faster.
The workhorse of Iran's enrichment program is the P-1 centrifuge, which is run in cascades of 164 machines. But Iranian officials confirmed in February that they had started using the IR-2 centrifuge that can churn out enriched uranium at more than double the rate.
Iranian state television did not say if the installation of the 6,000 new centrifuges included the older P-1 or the advanced IR-2 centrifuges.
Diplomats in Vienna told The Associated Press on Thursday that Iran has assembled hundreds of advanced centrifuges at Natanz.
One diplomat said more than 300 of the centrifuges have been linked up in two separate units in Iran's underground enrichment plant and a third was being assembled. He said the machines apparently are more advanced than the thousands already running underground, suggesting they could be the sophisticated IR-2 centrifuge.
But a senior diplomat said that while the new work appeared to include advanced centrifuges, they were not IR-2s. Both diplomats are linked to the Vienna-based International Agency for Atomic Energy, the U.N. nuclear watchdog, but asked for anonymity because their information was confidential.
A total of 3,000 centrifuges is the commonly accepted figure for a nuclear enrichment program that is past the experimental stage and can be used as a platform for a full industrial-scale program that could churn out enough enriched material for dozens of nuclear weapons.
Iran says it plans to move toward large-scale uranium enrichment that ultimately will involve 54,000 centrifuges.
The U.N. has passed three sets of sanctions against Iran for its refusal to suspend enrichment.
On Tuesday, China said it would host a meeting of officials from the five members of Security Council and Germany, as well as the EU, to discuss ways to restart negotiations with Iran on its nuclear program.