The order issued by city engineers was formal and clear: demolish the 76-year-old Figueroa-Riverside Street Bridge by early spring, clearing the way for completion of a new bridge across the Los Angeles River.

The order came after the Los Angeles City Council granted authority to tear down the old concrete and metal truss bridge between San Fernando Road and the 5 Freeway, a connection between the communities of Elysian Valley and Cypress Park.

Now, with the $68-million replacement being built mostly with federal funds, a group of architects and activists have launched a campaign to spare the old bridge and transform it into an "elevated green space" for bicyclists and pedestrians.

"This is an opportunity to improve the quality of life here and preserve an elegant structure that played an indispensable role in the development of Los Angeles," architect Kevin Mulcahy, a leader of the effort to save the bridge, said Friday while leaning on the bridge's sculpted concrete railing with its incised rounded arches.

Those aligning with Mulcahy include nonprofit groups Enrich LA and Los Angeles Walks and neighborhood councils in the communities of Lincoln Heights, Cypress Park, Arroyo Seco and Elysian Valley. Their mission is to save the 170-foot stretch of the bridge that spans the river.

But Deborah Weintraub, interim city engineer in the Los Angeles Bureau of Engineering, said it is "very late" to alter course.

The final phases of building the new bridge anticipated removal of the old one, she said. Reconfiguring the final construction work would require negotiating a new construction plan with the contractor, Flatiron Construction Corp. of Firestone, Colo., adding costs of $2.5 million to $5 million.

The additional costs would probably not be funded by the federal government, she said, and "would not include the expense of designing and constructing the green space they want."

"We admire the passion of these preservationists — they've put their heart and soul into this campaign," Weintraub said. "But we're past the point of no return on this project."

As for waiting too long to raise objections, the activists note that the City Council's 2006 decision to remove the old bridge was based on its structural deficiencies and a presumption that a new one would literally take its place. Years later, however, the new bridge was sited far enough upstream to allow both structures to stand independently.

"That's when we decided to ask the question: why does the old bridge have to go?" said Rick Cortez, owner of RAC Design Build in Elysian Valley.

The Figueroa-Riverside Street Bridge was built in 1927 near the confluence of the Los Angeles River and the Arroyo Seco as a reinforced single-arch concrete structure featuring five pairs of decorative octagonal-shaped fluted light posts topped by lanterns.

Damage to the bridge from flooding in 1937 and subsequent landslides along adjacent Elysian Park hillsides required removal of its arch span and upper deck.

In 1939, the lower portion was retrofitted with a metal truss, signaling a return to steel bridge construction that had been all but abandoned during the city's aggressive bridge building program at the turn of the century. The use of steel construction dominates bridge building in Los Angeles to this day.

The city designated the bridge a city historic monument in 2007, conferring a status that ordinarily preserves a structure. But in this case, the monument designation occurred one year after the City Council voted on its replacement, rendering its historic status largely symbolic.

But because of that status, the city will thoroughly document its demolition, Weintraub said.

Protecting the bridge has become a full-time effort for Cortez and Mulcahy, and their tours of the structure follow circuitous routes through dense brambles and homeless camps. On Friday, a tour ended in a well-hidden complex of subterranean galleries and arches decorated with thousands of glistening examples of graffiti art at the western end of the bridge.

Los Angeles has taken steps to preserve bridges from its earlier years, mostly those that are among the nearly two dozen gems designed in the 1920s and 1930s by Merrill Butler, who served as engineer of bridges and structures for the Bureau of Engineering for nearly 40 years.

Mulcahy acknowledges that the Figueroa-Riverside Street Bridge is not a Butler bridge, but notes: "Museums are not filled with the works of a single artist. Nor should examples of the architectural industrial history of our city."

louis.sahagun@latimes.com