By Daniel Trotta

(Reuters) -Los Angeles commuters appeared to be heeding warnings to stay off the roads on Monday morning after a weekend fire forced the indefinite closure of a downtown stretch of the busy Santa Monica Freeway, raising the prospect of an extended traffic nightmare.

About 300,000 vehicles use the freeway daily, and the area is often clogged even under normal circumstances.

Despite the concerns raised by the fire, which charred support columns on a freeway overpass, Laura Rubio-Cornejo, general manager of the Los Angeles Department of Transportation, said motorists appeared to be heeding the warnings of delays.

"I am someone pleased to say that the congestion was a little better than normal," Rubio-Cornejo said. "However, if you don't need to be in downtown Los Angeles, please avoid those trips."

Even so, roads in and around downtown Los Angeles were jam-packed, according to local media, and gridlock could result if there were even minor traffic accidents, as hundreds of thousands of motorists hunt for alternative routes.

California Governor Gavin Newsom on Sunday proclaimed a state of emergency in Los Angeles County in order to expedite repairs to the freeway. While touring the damage, the governor vowed to get the highway reopened as quickly as possible.

"It is all hands on deck for everybody in state government," said Toks Omishakin, California's transportation secretary, during the press conference on Monday morning.

"Our maintenance and structural folks are doing work, getting core samples of the structure to see what full condition the bridge is in and the columns," he said.

A section of freeway, also known as the east-west Interstate 10 - or "the 10" in local parlance - is closed in both directions at a point between Interstate 5 and Interstate 110, two other major freeways vital to getting around Los Angeles, where traveling by car is the norm.

The closure was likely to last several days or longer depending on how badly the bridge is damaged, Los Angeles Mayor Karen Bass said.

Smoke billowed into the night sky after the fire broke out shortly after midnight on Saturday. It took 164 firefighters from 26 fire companies several hours to put out the blaze, the Los Angeles Fire Department said.

The fire spread through at least two storage yards underneath the freeway that held stacks of wood pallets and containers, the fire department said.

No injuries were reported and the cause was under investigation, the fire department said.

The governor's proclamation is intended to speed the repair work and cleanup. It requires the California Department of Transportation (Caltrans) to pursue aid from the federal government if appropriate.

Caltrans engineers are assessing damage to both the support columns and the deck of the freeway to develop a repair plan.

Bass said on Monday that thousands of people were notified through mobile alerts, the Nextdoor app, and an Instagram Live about the closures and alternate routes.

"Unfortunately, there's no reason to think that this is going to be over in a couple of days," Bass said.

L.A. transportation officials urged drivers to transfer to other freeways instead of getting off 10 near the damaged area to get around the closure.

Commuters were also urged to use the Metrolink transit system, which said it added six additional trains on its San Bernardino Line.

(Reporting by Daniel Trotta; additional reporting by Rich McKay in Atlanta; Editing by Noeleen Walder, Sandra Maler and Jonathan Oatis)