Women crowd into a second-floor gallery, hidden behind smoked glass so they will not distract the men below.
Small children peer from any spare space. Some even sprout from openings in the ductwork along the ceiling.
They all have come to catch a glimpse of the spiritual leader of the Lubavitch sect, Grand Rabbi Menachem Mendel Schneerson, the elfin 90-year-old scholar they call "the rebbe."
And they believe the frail, ailing Rabbi Schneerson is the Messiah.
"The attachment that a Lubavitcher feels toward the rebbe is so intense, so encompassing, the magnetism is so great, you always want to get another look," said Rabbi Abraham Flint, a spokesman for the Crown Heights Jewish Community Council.
The stampedes to see the beloved Rabbi Schneerson have become even more urgent in recent months as the Lubavitch sect, the most prominent and influential Hasidic movement in the world, has become infused with Messianic zeal.
Most Lubavitchers believe that any day the apocalypse will occur, life on Earth as we know it will end, and the Messiah will emerge from this rundown Brooklyn neighborhood to redeem mankind.
"We've accepted the rebbe as Messiah," said Rabbi Flint. "We're waiting for God to give him the word."
Rabbi Schneerson, for his part, has not had much to say on the matter. Paralyzed after suffering a stroke last March, he cannot speak.
Therein lies the difficulty for Lubavitchers. They are left to interpret the rebbe's silence and his limited gestures on the occasions when he is brought out in public, brief moments that drive his followers into a frenzy of excitement.
Does the rebbe acknowledge he is the Messiah? Or is he merely nodding in his wheelchair, remote and unaware?
"As he becomes closer to death, the drama increases, intensifies," said Jerome R. Mintz, an Indiana University professor and author of a book on Hasidim.
"Will the Messiah arrive? Will Rabbi Schneerson die? It's a cosmic drama. Their hearts are very much involved in it."
To the outside world, Lubavitch (pronounced Luh-BAH-vitch) is known largely
because of the recent racial strife in Crown Heights, a tired neighborhood where about 20,000 Hasidim live among a larger population of African Americans.
The neighborhood was convulsed by three days of riots in 1991 after a car in Rabbi Schneerson's entourage accidentally killed a 7-year-old black child. Sparked by rumors and resentment, blacks attacked Jews and their property. A rabbinical student was surrounded by a mob and stabbed to death.
But racial discord is a secondary concern to Lubavitchers. To them, nothing matters more than the rebbe.
Rabbi Schneerson's photograph appears on the wall of every Lubavitch home and every Lubavitch business in Crown Heights. His piercing blue eyes, in dozens of paintings and photographs, stare out from the window of Judaica World on Kingston Avenue, the main commercial thoroughfare in Crown Heights.
His followers in the Israeli military - Lubavitchers are said to number more than 200,000 worldwide - paste the rebbe's photograph on their tanks.
Lubavitchers have built an exact replica of the Brooklyn world headquarters in the Israeli desert so that when the Messiah arrives he will have a familiar home in the Holy Land.
Rabbi Schneerson is the seventh leader of Lubavitch, which was founded in the 18th century and named after a town in what is now Belarus. As in all Hasidic sects, which flourished in Eastern Europe, leadership passed down through family members.
Hasidic worship is charismatic, emotional and mystical. It employs dancing, singing and drinking. Hasidim - which means "pious ones" - are traditional in their behavior and dress. Men do not shave their beards. Married women shear off their hair and customarily wear wigs.
Lubavitchers, unlike members of other more isolated Hasidic sects, view their mission as to redeem secular, wayward Jews. Lubavitch proselytizers travel about in mobile homes called "mitzvah tanks" - mitzvah means "good deed" - to urge mainstream Jews to become more Orthodox.
In the 1930s in Europe, the recognizable Hasidic Jews were easy targets of ethnic attacks. While some Hasidic sects were annihilated, the Lubavitchers assured their survival by relocating to Brooklyn in 1940.
Rabbi Schneerson, a descendant of the third rebbe and son-in-law of the sixth rebbe, assumed leadership in 1950 when his father-in-law died.
Lubavitchers believe the rebbe possesses mystical powers. Before Rabbi Schneerson's stroke, a Lubavitcher would consult the rebbe on any personal decision - marriage, a new job, a new home. His advice was accepted unconditionally. They still send him letters and receive responses from his staff.
His influence stretches far beyond Crown Heights. In 1988, a few words from the rebbe at the last minute swung five seats in the Israeli election to an
obscure right-wing party.
The rebbe's every public word and movement have been watched, recorded and studied. At informal gatherings called farbrengen, he spoke for hours extemporaneously.
Every word was captured on video and audiotape, except for his appearances on the Sabbath, when it is prohibited to use any recording device, even a pen. On those days, his speeches were memorized by teams of special rabbis, who later transcribed them.
Lubavitchers say it has been clear since Rabbi Schneerson assumed leadership that he had great Messiah potential.
Rabbi Schneerson told the faithful that "the goal of our generation is to bring about the coming of the Messiah." He also noted that he was the seventh generation of his family to lead the sect - the same number of generations separating Abraham and Moses.
To the faithful, Rabbi Schneerson's words are not to be taken lightly.
"No phrase is incidental," said Rabbi Flint. "Every single word is considered meaningful. We don't always understand it, but every word is perfect."
Lubavitchers attach great meaning to the fact that Rabbi Schneerson never had children. There are no heirs to the dynasty. And yet no one appears to be concerned about who will take over.
The question of a successor "is not even a matter of consideration," said Rabbi Yehuda Krinsky, one of Rabbi Schneerson's executive secretaries and Lubavitch's chief spokesman.
"You can chalk that up as another unusual aspect of Lubavitch."
The history of Judaism is marked by episodes of Messianic fervor.
In the 17th century, thousands of Jews prepared to return to Jerusalem to follow a Turkish rabbi, Sabbatai Sevi. But he converted to Islam to avoid execution, leaving his followers crestfallen.
The most famous example was the advent of Jesus, who Jews believe was a false Messiah. "That kind of put Judaism on the back burner of monotheistic religions," said Rabbi Allan Nadler, the director of the YIVO Institute for Jewish Research in New York.
So most Jewish leaders regard messianism as potentially disruptive. One rival Hasidic rabbi in Israel, Eliezer Schach, has denounced Rabbi Schneerson as a "false Messiah" and called Lubavitchers "heretics and eaters of unclean flesh who have no ties to the Torah."
"The Lubavitchers are very loud and have a high profile," said Rabbi Nadler. "But when all is said and done, they're just one small sect in the Jewish community."
Lubavitch leaders argue that Rabbi Schneerson has never claimed to be the Messiah. They point out that in the past, when he was able to speak, he would admonish those who proclaimed publicly that he was the savior.
But all that changed when the rebbe lost his voice, and his followers raised their voices with vigor renewed.
The central sanctuary of the Lubavitch world headquarters is a humble, scruffy room carved out of the interior of several converted apartment buildings. The paint is chipped, molding is missing, linoleum is worn away. Candles are set in dented cake pans, not elaborate candelabra.
Aesthetics do not matter in this worship service. The rebbe is the whole show.
One Sunday last month, the synagogue was crowded to capacity, this time for the special anniversary service being transmitted by satellite to Lubavitchers in London, Paris, Jerusalem, Tel Aviv, Moscow, Hong Kong, Sydney and Melbourne.
Rabbi Schmuel Butman, who had spent the week drumming up excitement about the service, said its purpose was "so all of us can announce that God should give our rebbe the directive to reveal the Messiah."
Rabbi Butman's statements led to heightened excitement and hope that the event would be a "coronation" of the Messiah.
But Lubavitch elders, concerned that the secular news media were mocking the Messianic fervor, admonished the faithful to contain their zeal.
Since falling ill, Rabbi Schneerson has made his appearances while sitting in a chair that has been lifted onto a balcony overlooking the synagogue. He is concealed behind a beige curtain, which opens for only a few minutes.
The assembled Hasidim erupted in song the instant the curtain began to open. The song is at the heart of the dispute over whether the rebbe acknowledges he is the Messiah. It is a prayer in Hebrew, and the cadence grew faster and faster:
"Long live our master, our teacher, our rabbi. King, Messiah, forever and ever."
Rabbi Schneerson, when he could speak, would halt the singing. But one Sunday last month, as with all his recent appearances, he made no apparent gesture to stop the song.
Rather, when he moved his left hand - the only hand he can move - the Lubavitchers interpret the sign as consent and pick up the cadence even more.
"If he doesn't want something, he knows how to stop it," said Chaim Halberstam, an audiovisual expert who tapes all the rebbe's non-Sabbath appearances.
Rabbi Krinsky, the Lubavitch spokesman, said it was unfair to interpret the rebbe's limited gestures as agreement.
"The fact that he might make a gesture with his hand, it's what he always did with his hand," said Rabbi Krinsky. "We're not even sure if he's doing this because of the words or because of the melody, because it's kind of a lively melody."
The question, of course, will be settled only with the rebbe's death, an occurrence that most Lubavitchers believe will never happen.
"Other religions put themselves on the line in the hereafter," said Rabbi Flint. "We put ourselves on the line in the here and now."