Mahmoud Ahmadinejad likened the offer to accepting candy in payment for gold.
"They say we want to give Iranians incentives, but they think they are dealing with a four-year-old, telling him they will give him candies or walnuts and take gold from him in return," he told a crowd in the central city of Arak on Wenesday.
In the speech, broadcast live on the state television, he said the EU should not "force governments and nations who are signatories to the atomic Non-Proliferation Treaty to pull out of it".
Britain, France and Germany plan to offer the light-water reactor as part of a package of incentives to Iran in return for the freezing of its uranium enrichment programme, diplomats have said.
Nuclear experts believe it is more difficult to use the light-water reactor to develop nuclear weapons than a heavy-water plant.
Iran says it is enriching uranium as part of a civilian nuclear programme to satisfy the country's energy needs.
However, the US and EU accuse Tehran of using this programme to cover up its pursuit of nuclear weapons.
EU diplomats said on Tuesday that they would be surprised if Iran accepted the offer, but would take rejection as a confirmation of their suspicions regarding Tehran's nuclear aspirations.
The EU trio first proposed offering Iran light-water technology in 2005 after two years of negotiations. At the time, the Iranians said the offer lacked specific incentives.
Diplomats said the new offer would be more specific, partly because they were now confident of full US support.
"They think they are dealing with a four-year-old, telling him they will give him candies or walnuts and take gold from him in return"
The offer is being made, they said, to demonstrate to Russia and China, the most sceptical members of the UN security council, that they were not depriving Iran of the opportunity of a civilian programme.
Also on Wednesday, a high-level meeting on Iran has been postponed while the United States lobbies other UN Security Council permanent members to harden proposed penalties if Tehran does not give up uranium enrichment, diplomats said.
The London meeting of senior representatives from the five permanent council members and Germany was to have been on Friday.
But diplomats told The Associated Press that it had been moved to Tuesday or Wednesday to allow more time for phone discussions on incentives and penalties to be offered to Tehran, a diplomat said, demanding anonymity because of the confidential nature of the information.