The Jewish Sabbath that begins at sundown tonight will be a particularly significant one for thousands of observant Jews in a wide swath of Manhattan. Today marks a major, southward expansion of an eruv — a symbolic boundary that allows certain actions, like carrying things or pushing a stroller, which would otherwise be forbidden under Jewish law.

The eruv began on the Upper West Side in 1994 and was extended eastward, to the East River, in 2004. Until now, its southern boundary ran through the West and East 50s. About two years ago, a group of rabbis and congregations in Manhattan began discussing how to extend part of the eruv southward, to Houston Street between First and Sixth Avenues. That expansion takes effect today.

The expansion of the eruv involved stringing translucent fish wire across the tops of lampposts. The wire is high enough off the ground as to be virtually imperceptible to passers-by. (The wires in the original Upper West Side eruv were taken down each Thanksgiving to allow the giant Macy’s Thanksgiving Day Parade balloons to pass unimpeded.)

“We’ve been working with a team of rabbis who are experts not only in the Talmudic law but also in the practicalities of how to do it in a city,” said Rabbi Yehuda Sarna of the Edgar M. Bronfman Center for Jewish Student Life at New York University, a main organizer of the effort.

The cost of the expansion came to more than $100,000, raised from members of 11 participating congregations, from Congregation Adereth El (an Orthodox synagogue) to the Central Synagogue (a Reform temple), Rabbi Sarna said.

The eruv will be inspected once a week by a rabbi — “I guess you could basically call him the eruvitect,” Rabbi Sarna said — to confirm that the circuit is intact. If there is any problem, the rabbi will call a construction company to repair any frayed or toppled section of wire.

The expansion has generated considerable excitement.

“We’re all very proud — all Jews in this neighborhood are proud — to preserve the Jewish traditions,” said Rabbi Gideon Shloush of Congregation Adereth El, on East 29th Street, which has been active in the effort to extend the eruv. “It’s phenomenal. It’s something special and long-awaited.”

Because the Sabbath is a day of rest, activities like carrying, cooking or traveling are proscribed. An eruv (the term, meaning “mixture,” technically applies to an array of practices, not just a symbolic boundary) designates an entire area as “one domain,” or a shared space, according to Rabbi Sarna. “It domesticates the space, makes it like one home, where everyone is a family,” he said.

The expansion covers parts of Midtown and Greenwich Village, along with neighborhoods like Murray Hill and Gramercy Park. “Really what the construction of the eruv points to is the excitement around the growing Jewish presence in these areas,” Rabbi Sarna said. “Some of these areas the most hip and the most commercial in the city and the United States.”

As with any religious practice, however, the eruv is not universally recognized, even among observant Jews. As a Times article in March 2006 pointed out, the extension only goes south to Houston Street and does not extend into the Lower East Side – one of the historic centers of Jewish life in America — because some Orthodox rabbis there do not believe that Manhattan’s traffic patterns and street layout allow for valid eruvs.

Rabbi Sarna, while acknowledging that some rabbis do not acknowledge the eruv, said the process had brought together Jews of different movements and beliefs, noting that several Conservative and Reform congregations endorsed the effort.

“I always thought eruv was about Orthodox Jews who followed Shabbat very strictly gaining the ability to carry in certain areas,” he said. “But what I came to understand is that the Talmud says that the real reason the eruv was instituted was to open the ways to peace.”