"No one can take away our nuclear technology. The Iranian nation has obtained it and will preserve it. Some are against the Iranian nation's development," he said in a televised address to mark the start of the Iranian year on March 21.
He spoke as senior foreign affairs officials from the five veto-holding U.N. Security Council powers and Germany were to meet to try to break the impasse over Iran's nuclear ambitions.
The Council has been unable for nearly two weeks to issue a statement telling Iran to halt uranium enrichment efforts which the West believes are a cover for bomb making. Iran says its nuclear programme is simply for generating electricity.
Ahmadinejad said Iran would resist efforts to undermine its nuclear programme just as it pursued the nationalisation of its oil industry last century against the wishes of Western powers
"A while ago, they were against the nationalisation of our oil industry. They were saying that this (oil) smelt bad and is useless and you don't need it. That day the Iranian nation resisted, and today they are saying the same things about nuclear energy," he said.
"The Iranian nation will resist with God's help," he added.
Iran had previously agreed to suspend its uranium enrichment activities under an agreement with Britain, France and Germany. That deal broke down last year and, in February, Iran started tests on enriching uranium.
"They were hoping through propaganda and psychological war to take this (nuclear technology) away from us. I am suggesting to them to learn their lesson and apologise to the Iranian nation," he said.
"They should be sure that if they continue, they will regret continuing to make baseless propaganda against Iran," he said without elaborating.