(Adds Khamenei comments, currency, stock market)
DUBAI, Aug 28 (Reuters) - Iranian Supreme Leader Ayatollah Ali Khamenei said on Wednesday U.S. intervention in Syria would be "a disaster for the region", the ISNA state news agency reported, as Western powers made plans to hit Damascus over a chemical weapons attack.
After supporting Arab uprisings across the Middle East and North Africa in 2011 as examples of what Khamenei called an "Islamic awakening", Tehran has steadfastly supported the secular President Bashar al-Assad, its main strategic ally in the Middle East, against a two-and-a-half-year-long rebellion.
"The intervention of supra-regional and foreign powers in one country will have no result other than lighting a fire and increase the hatred people have for them," the ISNA agency quoted Khamenei as saying.
"This lighting of a fire is like a spark in a gunpowder magazine whose dimensions and consequences are unknown."
Iran is concerned that if Assad were overthrown, he would be replaced by either allies of the West or by radical Sunni Islamists tied to Saudi Arabia, both regarded as hostile by Shi'ite Iran. Syria is also a conduit for Iranian supplies to Shi'ite Hezbollah militants in Lebanon.
"Any intervention and warmongering will certainly harm those who start this fire," Khamenei said. "If such an action is taken, the Americans will certainly be harmed just like in their interventions in Iraq and Afghanistan."
The Iranian currency, the rial, dropped by 4 percent against the dollar and the main Tehran stock exchange index fell by 2 percent on Wednesday on what analysts said were worries about the repercussions for Iran of any Western strikes on Syria.
Iranian officials have condemned the use of chemical weapons - deployed against its troops during the 1980-88 Iran-Iraq war - but have blamed Syrian rebels for the Aug. 21 poison gas attack that killed hundreds in the embattled suburbs of Damascus. (Reporting by Yeganeh Torbati; Writing by Jon Hemming; Editing by Mark Heinrich)