President Obama declared on Friday a large chunk of the San Gabriel Mountains a national monument during the final day of a Southern California visit that will include a signing ceremony in the wilderness northeast of downtown Los Angeles.

Some 350,000 acres of land -- from Santa Clarita to San Bernardino -- was set aside Friday during a ceremony at Frank G. Bonelli Regional Park in the San Gabriel Valley community of San Dimas. The rugged moutains provided a backdrop for the event, at which Obama described the "magnificent" mountains as an important part of the nation's history.

"There haven't been enough resources to manage and maintain this area the way it deserves," Obama said. "We have a responsibilty to be good stewards of those landscapes for future generations."

The mountains' steep canyons and soaring peaks are home to several archeological and culturally significant sites, such as rock art, and Mount Wilson Observatory, where William Hubble discovered galaxies. The area also provides a sweeping view toward downtown Los Angeles and a stunning backdrop for football games at the Rose Bowl.

"Within these hills lies millennia of history including the ancient rock art of native peoples, the first Americans," Obama said. "It was here at the Mount Wilson Observatory that Edwin P. Hubble showed the universe to be ever expanding, and it's where astronomers still explore the mysteries of space."

Supporters say it will give much needed status to the area, protecting water access rights and public recreation, while leaving the land pristine. The U.S. Forest Service, always strapped for cash, has not always succeeded in this, spending more than half its annual budget on fire suppression.

The move will provide recreational opportunities for millions of people -- minorities and children in particular -- who live in Los Angeles County, one of the most disadvantaged areas in terms of access to open space, according to backers of the plan.

"We heard from the community that for a lot of urban families this is their only big, outdoor space," Obama said. "And too many children in LA County, especially children of color, don’t have access to parks."

Not everyone is on board.

"I don't see how that is going to preserve the mountains in the way that they say it will," Erin Letarte said.

Letarte is part of a small, but vocal opposition. Residents and some business owners are furious that politicians went to the President after a bill to turn the land into a "national recreation area" stalled in Congress.

Letarte said members of the public were cut out of the process.

Los Angeles Mayor Eric Garcetti said in a statement that the move will "ensure its resources are managed properly and protected for future generations to enjoy."

"The San Gabriel Mountains are a vital natural and cultural resource," he said.

A 2003 study by the National Park Service said the region deserves special attention and protection, especially considering that the forest and rivers provide an essential habitat for endangered and sensitive species. As national monument, the San Gabriel Mountains northeast of Los Angeles are expected to benefit from additional resources, such as increased maintenance, new picnic sites and restrooms, more rangers, and improved educational programs for children.

More than 15 million people live within 90 minutes -- depending on Southern California traffic -- of the San Gabriel Mountains.

Foundations such as the William and Flora Hewlett Foundation, the Wyss Foundation, the Annenberg Foundation, and the California Endowment are establishing the San Gabriel Partnership Fund to support such projects. Together, they have already raised $500,000.

Obama has used his authority under the federal Antiquities Act to create or expand 12 other national monuments nationwide. Last month, he created the world's largest marine preserve by expanding the Pacific Remote Islands Marine National Monument in the south-central Pacific Ocean.

NBC4's Jonathan Lloyd contributed to this report.