People take pictures of the Golden Gate Bridge as smoke from wildfires covers the San Francisco skyline Tuesday, Oct. 10, 2017, in this view near Sausalito, Calif. An onslaught of wildfires across a wide swath of Northern California broke out almost simultaneously then grew exponentially, swallowing up properties from wineries to trailer parks and tearing through both tiny rural towns and urban subdivisions. (AP Photo/Eric Risberg)

SAN FRANCISCO - Millions of Californians on Tuesday saw their lawns covered in ash and air filled with smoke as firefighters battled ferocious wildfires in the northern and southern parts of the state.

The fast-moving blazes produced thick, grey clouds that hovered over densely-populated areas in Northern and Southern California, forcing many schools to keep students inside and air quality agencies to issue smoke advisories calling on residents to limit outdoor activity.

At Disneyland, visitors snapped photos of hazy, orange skies late Monday that gave an ominous glow to a theme park already decked out for Halloween. Ash fell like snow over seaside Southern California communities more than a dozen miles from the hillside neighbourhoods where the fire raged.

"My eyes tear and it's uncomfortable for me to breathe," said Yolanda Ramos, 66, who had driven to Santa Ana from her Los Angeles County home to visit her 91-year-old father. "I wanted to take my father out in the wheelchair, and they said no."

At least 15 people have been killed and as many as 2,000 homes and businesses destroyed in an onslaught of fires stretching across Northern California.

The West this year has been hit hard by dozens of fires that have blanketed the air with choking smoke, prompting officials to issue air quality advisory alerts throughout the region.

The fires in Napa and Sonoma counties, home to dozens of world-renowned wineries, sent smoke as far south as San Francisco, about 60 miles (96 kilometres) away. Air quality concerns prompted schools throughout the San Francisco Bay Area to cancel outdoor football, soccer and other sports practices.

The Bay Area Air Quality Management District issued a warning saying the wildfires north of San Francisco are causing very unhealthy air quality throughout the region, and advised residents to stay inside if possible and keep windows and doors closed.

San Francisco officials put air filters in four public libraries for those seeking relief from the smoke.

Some workers in San Francisco's financial district wore masks as they went to the street from their offices.

To the south, the wind-driven brush fire that burned 12 square miles (31 square kilometres) in northeastern Orange County led officials to close more than a dozen schools. Schools as far away as Long Beach limited outdoor activity as a precaution or have kept children inside for physical education and recess.

"We're trying to keep them indoors," said Annie Brown, a spokeswoman for Irvine Unified School District. "It's kind of like a rainy day."

In the 24 hours since the fire began, Orange County health officials recorded a 7.5 per cent uptick in emergency respiratory transports to emergency rooms compared with the daily average for such cases last week. Most of these cases were related to asthma.

The South Coast Air Quality Management District issued an advisory in effect through Wednesday morning that warns the air quality may be unhealthy in large stretches of Orange and Riverside counties.

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Taxin reported from Santa Ana, California. Associated Press writer Paul Elias in San Francisco contributed to this report.

News from © The Associated Press, 2017