"Security forces have restored security in the area but the parade has totally been disrupted," a reporter on the scene for Iranian state television said by phone in a live broadcast.
"People have been killed but we have no figures yet."
Zarif on Twitter said that the gunmen were "terrorists recruited, trained, armed & paid by a foreign regime." He did not immediately elaborate. However, Arab separatist groups in the region have launched attacks on oil pipelines there in the past and Iran.
The separatists also accuse Iran's Shiite theocracy of discriminating against its Sunni Arab citizens. Iran has blamed its Mideast archrival, the Sunni kingdom of Saudi Arabia, for funding their activity.