The trial is the first in the case of the January incident, when Iranian protesters stormed the Saudi Embassy in Tehran and a consulate in Mashhad after Sunni-ruled Saudi Arabia executed Sheikh Nimr al-Nimr, a prominent Shiite cleric and dissident. Iran views itself as the defender of the world's Shiites.
The attacks on the diplomatic missions led to the severing of diplomatic ties between Saudi Arabia and Iran, regional rivals that back opposite sides in the civil wars in Syria and Yemen.
ISNA further said the defendants claimed they were not aware their gathering was illegal. One reportedly said he was repentant, after learning that Iran's Supreme Leader Ayatollah Ali Khamenei was not happy about the incident.
Last month, Iranian President Hassan Rouhani called for "transparency" in the trial and reiterated earlier remarks that the attacks on Saudi missions went against Iranian laws. In April, Iran said 48 people, including four clerics, would be tried in relation to the storming of the diplomatic missions.