PALA MESA (CNS) - Firefighters battled a fast-moving wildfire that tore across thousands of acres near Fallbrook and was moving toward Oceanside as it frstroyed dozens of homes and threatened hundreds of others, injured several people, closed roads and schools and prompted widespread evacuations.

At least 20 structures were destroyed and 12 damaged, according to Cal Fire Capt. Nick Schuler.

The wind-driven blaze erupted for unknown reasons about 11:25 a.m. Thursday just west of Interstate 15 and north of Lilac Road in Pala Mesa amid gusty, arid weather. By 8 p.m., the last time authorities updated the size of the blaze, flames had scorched 4,100 acres and crews had gained no containment on the fire.

At least six injuries have been reported, including a firefighter who dislocated a shoulder and one who was taken to a hospital for smoke inhalation. Three non-firefighters suffered burn wounds of unknown severity while another person suffered a case of smoke inhalation.

The flames moved quickly after the fire broke out, growing to 50 acres as ground and airborne crews labored to keep the blaze, dubbed the Lilac Fire, from becoming an out-of-control conflagration like others raging this week across Southern California.

By 2 p.m., the flames, driven by 35 mph winds, had spread over roughly 500 acres, were threatening an estimated 1,000 homes and had sent a thick pall of brown smoke to the southwest above the North County and out over the ocean, according to Cal Fire.

``This fire could make it all the way through Oceanside to the coast if it continues,'' Schuler said.

At least 23,000 residents from about 5,000 homes were under evacuation orders as of Thursday night, according to San Diego County officials. That number grew overnight and in the early morning hours today.

Many of the structures lost to the flames were residences in a mobile home retirement community near the origin point of the blaze, officials said. Television news helicopter footage showed rows of dwelling units at Rancho Monserate Country Club engulfed in fire or already reduced to smoldering rubble.

During a late-afternoon news conference Thursday, authorities urged residents of communities near the site of the blaze -- and across the San Diego region -- to remain vigilant in the face of ongoing threats of wildfire amid unseasonable weather conditions.

``We are nowhere near the end of this,'' county Deputy Chief Administrative Officer Ron Lane said.

``There are going to be more evacuations. There are thousands of homes that are within the path of (this fire), and every resident needs to take this very seriously and be prepared to take care of themselves and their families.''

Sheriff Bill Gore called on the public to ``stay alert'' and keep close tabs on the blaze from news coverage and local government bulletins issued online.

``I can't emphasize this enough -- if you feel threatened in any way, don't hesitate to self-evacuate,'' Gore said. ``Get your family out of harm's way. Go to some relatives, go to some friends or go to one of the (evacuation) sites ... . But don't wait for that ... deputy to knock on your door, because there's not that many of us out there.''

San Diego County was quick to proclaim a state of local emergency due to the blaze. The decree, issued in the mid-afternoon, will help make the region eligible for state and federal resources.

Later in the afternoon, Gov. Jerry Brown issued a state emergency proclamation for San Diego County.

Authorities issued an initial mandatory evacuation order for all areas south of state Route 76, west of Interstate15, north of Gopher Canyon Road and east of Mission Road/Vista Way. Among the evacuated sites were Bonsall High School, Sullivan Middle School and all neighborhoods along West Lilac Road.

Other sites under mandatory evacuation orders are:

-- Region south of Burma Road, east of Wilshire Road, north of North River Road and west of South Mission Avenue.

-- Area south of Reche Road, west of Interstate 15, east of Green Canyon Road and South Mission Road, and north of state Route 76.

-- Region south of Reche Road, north of Burma Road, east of the Camp Pendleton Eastern Fence Line and west of Green Canyon Road.

-- Area south of N. River Road, north of Bobier Drive, east of Melrose and N. Santa Fe and west of E. Vista Way.

-- Region south of Camp Pendleton Eastern Fence Line, north of N. River Road, west of Wilshire Road and east of Douglas Drive.

Residents of other nearby communities were advised to seriously consider retreating to safer areas as a precaution.

Authorities set up evacuation centers for the displaced at Bostonia Recreation Center in El Cajon, Carlsbad Forum in Carlsbad, East Valley Community Center in Escondido and at Oceanside and Fallbrook high schools.

A shelter at the Stagecoach Community Park filled to capacity Thursday night and evacuees were asked to go instead to the East Valley Community Center. The Oceanside High School shelter also filled to capacity this morning and evacuees were asked to go instead to a newly opened shelter at Palomar College.

Evacuees were also being directed to the Pala Casino. People with horses and livestock were advised to take their animals to shelter at the Del Mar Fairgrounds.

The California Highway Patrol closed down state Route 76 from Gird Road to Old Highway 395 and blocked off the latter route between West Lilac Road and state Route 76.

A contingent of about 700 firefighters from various area agencies was working to control the blaze as of late Thursday, Cal Fire Deputy Chief Dave Nissen said. Early this morning, Navy and Marine Corps aircraft are expected to join the other firefighting helicopters and air tankers assigned to the fire, Nissen told news crews.

Overnight, aircraft from the city of San Diego and Kern County were expected to conduct water drops, according to Nissen.

About 70 sheriff's deputies were working in the fire-ravaged area, handling road closures and providing security for evacuated homes, Gore told reporters.

The fire and the expected high Santa Ana winds today prompted the closure of campuses in at least 11 school districts including in Bonsall, Carlsbad, Fallbrook, Julian, San Marcos and Vista.

The cause of the blaze -- which broke out amid a National Weather Service ``red flag'' wildfire warning slated to expire Sunday night -- was not immediately clear.