Supreme Leader Ayatollah Ali Khamenei said Iran would not step back from its nuclear rights and his negotiating team had been set limits for talks over Iran's disputed nuclear programme, to resume in Geneva later on Wednesday.
Negotiators from Iran and six world powers meet for two days of talks in the Swiss city for the second time this month after a first round narrowly missed reaching an interim deal due to what diplomats said was Iran's insistence on its right to enrich uranium and French concerns over an Iranian heavy water reactor.
"We do insist that we will not step back one iota from our rights," Khamenei said in a speech to tens of thousands of volunteer Basij militiamen in Tehran, broadcast live on Iran's Press TV with English translation.
But Khamenei, the most powerful figure in Iran, said: "We do not intervene in the details of these talks. There are certain red lines and limits. These have to be observed. They are instructed to abide by those limits," he said.
"They should not be afraid of what the enemy says," he said.
In a likely reference to United Nations, U.S. and European Union sanctions imposed over Iran's nuclear activities, Khamenei said: "They intend to ratchet up the pressure on Iran. Iranians will succumb to no one under pressure.
"They should know that the Iranian nation respects all nations of the world, but we will slap aggressors in the face in such a way they will never forget it," Khamenei said, without referring to any specific country.
Recounting a long litany of what he said were U.S. and Western historic crimes and present efforts to gain hegemony in the Middle East, Khamenei did however say: "We want to have friendly relations with all nations, even the United States. We are not hostile to the American nation. They are like other nations in the world."
"Death to America," the militiamen chanted in response, repeating one of the main rallying cries for supporters of the Islamic Republic.
The leader criticised France. French President Francois Hollande assured Israel on Sunday that France would continue to oppose an easing of economic sanctions against Iran until it was convinced Tehran had given up any pursuit of nuclear weapons.
French officials, Khamenei said, were "not only succumbing to the United States, but they are kneeling before the Israeli regime".