Ayatollah Ali Khamenei also repeated his ban on direct talks with the United States about turmoil in the Middle East, saying US objectives in the region were utterly at odds with Iranian policy.
The comments, to Iran's ambassadors and other top diplomats, were Khamenei's first since his country joined on Friday international talks in Vienna on the four-year Syrian conflict.
In a wide-ranging speech on foreign policy Khamenei said Syria's people must choose for themselves who their leader would be, rather than US and other foreign powers deciding for them.
Top diplomats from 17 countries, as well as the United Nations and the European Union, had gathered in Austria to narrow their divisions over Syria's war, which has killed more than 250,000.
For the first time, the meeting brought together all the main outside players in the crisis, including Russia and Iran, key allies of the government of Syrian President Bashar al-Assad.
Iran and the US held direct talks in the past two years leading to a deal on the Islamic republic's nuclear programme but after the July 14 agreement Khamenei banned direct talks on regional issues.
"There's no point in other countries getting together and deciding about a system of government and the head of that state," he said Sunday.
"This is a dangerous innovation which no government in the world would accept being imposed on itself. The solution to Syria's problem is elections."
Khamenei said the military and financial support given to rebels fighting Assad, principally from Gulf states and the US, must be stopped.
