The Jewish community is about 27,000 strong, although the numbers have dwindled since 1979.
Before the Iranian Revolution, every seat in the synagogue was full for morning prayer, but in those turbulent years, half of Iran's Jews fled the country. Those who stayed behind hope they are not heading for a new crisis.
But the signs are ominous. Iran's hardline president has denied Hitler's holocaust and said Israel should be wiped off the map. Also, Israel refuses to rule out a strike on Iran's nuclear installations.
But in the shops and bazaars, Jewish merchants don't like to talk about the war of words.
"For the last 26 years we have very, very peaceful life with Muslims," one merchant says.
Instead, they stress that during the Iranian Revolution, Ayatollah Khomeini promised to protect them. But how do they feel when President Ahmadinejad says that Israel has no right to exist? "We know that all these things is politics, and they are not real," one Iranian Jew explains.
But the words were real enough to prompt the head of Tehran's Jewish community to do the unthinkable: write an outspoken letter to the president.
"The Holocaust is not a legend," Haroun Yeshayanei wrote. "We fear the values of the glorious Islamic Revolution will be overshadowed by racist sentiments."
Students at a Jewish school start the day singing allegiance to the Islamic Republic. Then they recite a Hebrew prayer.
It's a tricky balancing act. Today's politics may convince tomorrow's generation that it's simply too dangerous to be both an Iranian and a Jew.