Odd-even gasoline sales restrictions based on license plate numbers will begin at 12:01 a.m. Wednesday for nearly half of California's 15 million motorists in an effort to shorten waiting lines at service stations.

The restrictions will be imposed only in counties where local officials formally ask the governor to declare an emergency, Gray Davis, chief of staff for Gov. Edmund G. Brown Jr., said today at a hearing convened to let county officials and others discuss the fuel shortage. Before the hearing ended, three of California's biggest counties-Los Angeles, San Diego and Santa Clara-asked for such a declaration.

The three counties account for 6.7 million of California's 15 million motorists.

Under the odd-even restrictions, motorists with odd-numbered license plates could buy gasoline only on odd-numbered days, and motorists with even-numbered plates could buy it only on even-numbered days.

Meanwhile, an assocation of California service stations urged tourists to stay out of the state from May 17-20, when it plans a shutdown in protest of federal limits on the prices they can charge.

The Northern California Service Station Association advised tourists that gasoline probably will not be available at all on those days.

Many drivers throughout California arrived at stations today to find a long line of parked and locked cars ahead of them - left last night by others who walked or hitched rides to their cars this morning. At 6 a.m., the line to one open station in West Los Angeles stretched eight blocks.

Peter de Krassell of Pacific Palisades and his law partner, Jerry Cohen, galloped past the gas lines aboard two Arabian Horses.

"He's prepared to petition the city councils of Santa Monica, Beverly Hills and Los Angeles to temporarily rescind ordinances that prohibit horses on the streets," said Jack McGrath, a friend, as de Krassel rode through city streets. "There are enough people in the Palisades area who, if permitted, would certainly use horses to go to work, the bank, and do shopping."

The exact impact of the gasoline crunch on business in the Los Angeles area was difficult to ascertain. Area tourist attractions such as Disneyland Reported attendance was near or at normal.

In Nevada, by contrast, the weekend gasoline shortage hit some gambling resorts with a staggering 40 percent drop in room occupancy.

Las Vegas hotel-casinos in downtown's "Glitter Gulch" were particularly hard hit by the gas crunch.

State and local officials were to meet to consider a number of conservation plans, including a California style odd-even plan. But Bill Phillips, assistant to Gov. Robert List, said rationing was only a "last ditch" measure.