LOS ANGELES—An advocacy group accused officials Thursday of failing to obtain proper approvals when they allowed crews to begin work on a memorial to war heroes at a downtown site recognized as Los Angeles’ historic birthplace.

A lawsuit filed in Los Angeles Superior Court by The City Project argues that the Eugene A. Obregon Congressional Medal of Honor Memorial would alter parts of the El Pueblo de Los Angeles site that have important historical connections to the city’s Native American, Chinese American and other ethnic communities.

The suit also says the memorial, to be built within a grass-covered part of the historical site known as Father Serra Park, diminishes much-needed public space and demands that the city suspend work until a full environmental review is completed.

“The petitioners seek to preserve the historical integrity and green space of Father Serra Park and to have the proposed project moved to an appropriate site,” the suit says.

Crews have nearly finished the first stage of the memorial, which consists of a 30-foot long, 5-foot-high plaster wall covered with tiles bearing the names of nearly 3,500 medal recipients.

The memorial’s sponsors also plan a 20-foot high pyramidal monument paying tribute to the medal’s 40 Hispanic recipients.

A statue atop the stone structure would depict the memorial’s namesake, Marine Pfc. Eugene A. Obregon, coming to the rescue of a comrade during the Korean War. The 19-year-old Obregon died during the rescue.

City Project president Robert Garcia said he planned to ask a judge early Friday for an injunction halting further construction.

Garcia and leaders of other groups, including the state-chartered Native American Heritage Commission and the Chinese Historical Society of Southern California, had written the mayor’s office and other city officials in recent weeks to express concern over the memorial during its construction.

The letters cited evidence showing that Father Serra Park had been the location of a Native American village. They also noted that the park was part of the Spanish settlement that grew into the city of Los Angeles and that it was believed to be the place where 19 Chinese men were killed in the 1870s, when the area was the location of the city’s Chinatown.

Los Angeles Mayor Antonio Villaraigosa and other city officials were expected to attend a press conference Saturday unveiling the tile-covered wall, according to a release from the project’s organizers, the Eugene A. Obregon Congressional Medal of Honor Memorial Foundation.

Messages left seeking comment from the foundation, the mayor’s office and the city attorney’s office were not immediately returned.

The lawsuit was filed on behalf of the Concerned Citizens for South Central Los Angeles community organization, the nonprofit El Pueblo Park Association and a Tongva Indian group.

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Learn more about the struggle to save El Pueblo and Father Serra Park here.