"The Europeans' submitted proposals regarding the nuclear case are not acceptable for Iran," the official IRNA news agency on Saturday quoted Foreign Ministry spokesman Hamid Reza Asefi as saying.
"The proposals are unacceptable because Iran's right to enrich uranium is not included," he said.
Asefi said Iran would give a full answer to the EU's proposals on Saturday or Sunday.
The EU on Friday offered Iran a package of incentives for it to give up nuclear fuel work but also called an urgent meeting of the UN nuclear watchdog that could refer Tehran to the UN Security Council for sanctions.
The EU - represented by Britain, France and Germany - has been trying to find a compromise for two years between arch-foes Iran and the United States.
Hamid Asefi: The Europeans have
not kept their word
Washington accuses Iran of trying to covertly build a nuclear bomb, but Tehran denies the charge and says it has the right to convert and enrich uranium for power generation.
"I should say the Europeans have not honoured their commitments," Asefi said. "We have repeatedly said that any proposal should include Iran's right to enrichment."
Meanwhile, surprise election winner Ahmadinejad has been formally sworn in as the president of Iran on Saturday after he was approved, last Wednesday, by Iran's most powerful figure Supreme Leader Ayatollah Ali Khamenei.
"We want justice, peace and honour for all," he said after taking the oath of office on the Quran at a parliamentary ceremony.
"Managers of the Islamic government have no duty but to serve the people."
Ahmadinejad, 48, deeply loyal to the values of Iran's 1979 Islamic revolution, won a landslide election victory in June, replacing reformist cleric Mohammad Khatami, who was unable to stand for a third term.
Ahmadinejad stood firm on Iran's nuclear dispute with the EU.
"We are logical and respect international rules, but will not give in to those who want to violate our rights," said Ahmadinejad. "The Iranian nation cannot be intimidated."
On Friday, the United States explicitly accepted for the first time that Iran can develop civilian nuclear programmes, backing an EU proposal to allow Tehran to pursue atomic power in exchange for giving up fuel work.
In a compromise that completed a gradual shift in US policy, Washington acquiesced because it believes the EU offer has enough safeguards to prevent Iran from diverting its civilian work into making nuclear bombs.
The US accepts that Iran can
develop civilian nuclear projects
"We support the (Europeans') effort and the proposal they have put forward to find a diplomatic solution to this problem and to seek an end to Iran's nuclear weapons program," State Department spokesman Tom Casey told reporters.
The US acquiescence is in contrast with its stance in talks with North Korea, which it insists cannot have any nuclear development for fear Pyongyang would build atomic bombs under the guise of a civilian power program.
The shift also comes despite long-held US worries that allowing a civilian program could help Iran develop its nuclear technology and know-how so that, if it ever breaks any EU agreement, it would be closer to acquiring a bomb.
A US official said the EU offer helped allay American fears.
"There's a certainty and an ability to ensure that none of the nuclear fuel that would be involved is diverted to an illicit nuclear weapons program," said the official, who asked not to be named because he was not authorised to discuss details of the proposal.