By Lizbeth Diaz

TIJUANA, Mexico, Oct 22 (Reuters) - Raging wildfires in Southern California cloaked neighboring Mexican cities in a fog of suffocating ash on Monday, shutting schools and businesses and a major U.S.-Mexico border crossing.

The normally bustling city of Tijuana, just 20 miles (32 km) from San Diego County, where seven fires burned, virtually closed down as residents stayed indoors and those who ventured out walked around with paper masks over their mouths.

A thin gray ash covered houses and cars and blotted out the sun in the Pacific port of Rosarito and the city of Tecate, where U.S. and Mexican officials closed the busy border crossing.

"You can feel the heat of the fires from here. There's ash everywhere, it is falling like rain," said 33-year-old secretary Lorena Morales, who ran out to the pharmacy to seek anti-allergy medicines for her children.

The world's busiest land border crossing between Tijuana and San Diego remained open but many motorists sat uncomfortably for hours in the heat with their windows up and without air-conditioning to prevent ash coming in, witnesses said.

Southern California is in the midst of its driest year on record after rainfall just a fifth of average levels.

Authorities ordered at least 250,000 people to flee their homes on Monday because of the wildfires, which destroyed hundreds of buildings.

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