Iran's President Mahmoud Ahmadinejad has vowed that Iran will not halt work on its controversial nuclear programme.
Mr ElBaradei is holding talks with senior Iranian officials
He said the country would not back down "even one iota", despite mounting international pressure after it announced that it has enriched uranium.
The head of the UN's nuclear watchdog, Mohamed ElBaradei, is in Tehran for talks aimed at defusing the stand-off.
He said he hoped to convince Iran to suspend its uranium enrichment "until outstanding issues are clarified".
Western nations suspect Iran of wanting to develop a nuclear weapon, but Tehran insists its plans are for a peaceful, civilian energy programme only.
Speaking as Mr ElBaradei arrived in Tehran, Mr 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.
The BBC's Frances Harrison in Tehran says there is no sign of a compromise from Iran - but there is debate within the country about whether that is the right direction.
Reformists argue that having mastered enrichment, Iran is now in a strong position and can afford to make concessions to the West.
The US and Europe are pressing for sanctions against Iran, a step UN Security Council members Russia and China have opposed.
A senior Chinese arms control official, Assistant Foreign Minister, Cui Tiankai, is due in Tehran for talks on Friday.
The BBC's Daniel Griffiths in Beijing says China has so far kept a low profile but it is increasingly keen to be seen as a responsible, international player, and Iran is a perfect opportunity to strengthen those credentials.
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 risk isolation.
On his arrival in Tehran he said he was seeking "more active co-operation" between Iran and the IAEA.
NUCLEAR FUEL CYCLE
Mined uranium ore is purified and reconstituted into solid form known as yellowcake
Yellowcake is chemically processed and converted into a gas by heating it to above 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
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's rhetoric in recent days has been triumphalist, our correspondent reports. Nuclear officials are boasting they will now accelerate their work to produce nuclear fuel on an industrial scale.
One newspaper headline said the West was now "checkmated". Another said Mr ElBaradei was welcome to join Iran's nuclear celebrations.
If Iran decided to develop highly enriched uranium, it could take between three and five years to make enough for a single nuclear bomb, assuming that it mastered the technology, the International Institute of Strategic Studies reports.
But the IISS also says it could take as long as 10-15 years, depending on Iranian ability and intentions.