Iran’s supreme leader, Ayatollah Ali Khamenei, appeared publicly on Saturday night for the first time since the 12-day war with Israel began last month, Iran’s state media and his official website reported.

Mr. Khamenei issued a defiant video message last week, in his second public statement after the U.S. bombing of Iranian nuclear facilities. But his long absence from public life was without precedent during his three-decade rule, and had fueled speculations about his health and threats to his life.

After weeks of sheltering in a bunker, Mr. Khamenei on Saturday night attended an annual Shiite religious ceremony for Ashura, which was held at his compound.

The attendees appeared surprised. When Mr. Khamenei walked in, wearing a black clerical robe and his checkered kaffiyeh, the crowd of hundreds of men and women rose to their feet, thumping their fists in the air, and chanting, “Heydar, Heydar,” a Shiite battle cry.

Mr. Khamenei waved and sat on the side of the stage, videos on state television showed. He did not address the crowd, which included the vice president, minister of justice, and speaker of the Parliament, sitting on the floor. Mr. Khamenei was the only person perched on a chair.

In his brief video messages to the public, after the United States bombed Iran’s nuclear sites and after a cease-fire with Israel went into effect, Mr. Khamenei appeared weak, struggling to finish sentences in a raspy voice.