The hard-line leader also repeated his call for negotiations, saying the time for "bullying" had expired.
"The train of Iranian nation is without brakes and rear gear," the radio quoted Ahmadinejad as telling a gathering of Islamic clerics. "We dismantled the rear gear and brakes of the train and threw them away sometime ago."
Ahmadinejad's comments come a day before senior officials of the five permanent members of the U.N. Security Council — Britain, the U.S., France, China and Russia — and Germany were set to meet for an emergency summit in London to discuss measures against Tehran over its defiant nuclear stance.
The International Atomic Energy Agency last week reported that Iran had ignored a U.N. Security Council ultimatum to freeze its uranium enrichment program and instead had expanded the program by setting up hundreds of centrifuges.
In December, the Security Council imposed limited sanctions on Iran over its refusal to suspend enrichment and gave it a 60-day grace period to halt enrichment. That grace period expired Wednesday.
Iran has repeatedly refused to halt enrichment as a precondition to negotiations about its program. Enriched to a low level, uranium is used to produce nuclear fuel but further enrichment makes it suitable for use in building an atomic bomb.
The United States and several of its allies fear that Iran is using its nuclear program to produce atomic weapons — charges Iran denies, saying its aim is to generate electricity.
Ahmadinejad also said Western countries feel threatened by Iran's nuclear program because they feel their own powers are diminishing.
"The Westerners are not concerned about the existence and activity of ... centrifuges in Iran; they are concerned about the collapse of their hegemony and hollow power," the radio quoted Ahmadinejad as saying.