The men, mainly academics, professors and students, are members of the Islah Party, the Muslim Brotherhood group in Yemen. They were arrested during a Houthi security campaign in 2016.
The verdicts against the men can be appealed, the official said on condition of anonymity.
Amnesty International condemned the trial as a "mockery of justice," and urged the Houthis to immediately release the 30 men.
Saudi Arabia has been leading an Arab military coalition against the Iran-allied Houthi rebels in Yemen for more than four years in support of the exiled internationally recognized government of Yemeni President Abd-Rabbu Mansour Hadi.