UN nuclear watchdog chief Mohamed ElBaradei is in Tehran for talks aimed at defusing an international stand-off over Iran's nuclear activities.
Mahmoud Ahmadinejad says Iran's intentions are peaceful
He said he hoped to convince Iran to suspend its uranium enrichment "until outstanding issues are clarified".
But as he arrived, Iran's president again rejected demands to halt work on his country's nuclear programme.
There has been mounting criticism of Iran over its announcement that it has enriched uranium for the first time.
Speaking as Mr ElBaradei arrived in Tehran, Mahmoud Ahmadinejad said: "Our answer to those who are angry about Iran obtaining the full nuclear cycle is one phrase, we say: Be angry and die of this anger."
"We will not hold talks with anyone about the Iranian nation's right [to enrichment] and no one has the right to step back, even one iota," he added, the official IRNA news agency reported.
Western nations suspect Iran of wanting to develop a nuclear weapon, but Tehran insists its intentions are peaceful.
Mr ElBaradei, who heads the International Atomic Energy Agency (IAEA), is to report back to the UN Security Council at the end of this month on whether Tehran is complying with its demand to stop all enrichment activity by 28 April or face isolation.
On his arrival in Tehran he said he was seeking "more active co-operation" between Iran and the IAEA.
He said he wanted to discuss "how we can bring Iran in line" with demands by the international community that it cease enrichment and take "confidence-building measures".
Iran could be in a position to produce enough fissile nuclear material to make a nuclear bomb within three to five years, according to the International Institute of Strategic Studies.
Iran's deputy nuclear chief Mohammad Saeedi has said Iran intends to move toward large-scale uranium enrichment involving 54,000 centrifuges, signalling the country's resolve to expand its nuclear programme.
Iran announced on Tuesday that had succeeded in enriching uranium on a small scale for the first time, using 164 centrifuges, at a facility in Natanz.
UN Secretary General Kofi Annan said on Wednesday he hoped Mr ElBaradei would be able to persuade Iran to resume negotiations.
US Secretary of State Condoleezza Rice said the Security Council would need to take "strong steps" to maintain "the credibility of the international community on this issue".
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is converted into a gas by heating it to about 64C (147F)
Gas is fed through centrifuges, where its isotopes separate and process is repeated until uranium is enriched
Low-level enriched uranium is used for nuclear fuel
Highly enriched uranium can be used in nuclear weapons
UK Foreign Secretary Jack Straw said he was "seriously concerned", adding Iran's latest move "further undermines international confidence in the regime and is deeply unhelpful".
Germany and France also voiced their worries, and China said it was "concerned about the event and the way things are developing."
However, China's ambassador to the UN Wang Guangya urged all parties to "exercise restraint, act constructively and not to take action that might further aggravate the situation".
Russian Foreign Minister Sergey Lavrov said the move was "in the wrong direction", but warned against dramatising the situation and reiterated Moscow's firm opposition to any military action against Iran.
The US and Europe are pressing for sanctions against Iran, a step Security Council members Russia and China have opposed.