Gov. Edmund G. Brown Jr., reacting today to long lines of frustrated and sometimes violent motorists at California's gasoline pumps, announced an emergency plan to allocate remaining fuel.

That plan, an "odd or even" measure, could go into effect "Monday p.m. or early Tuesday," according to one of Brown's top aides.

The plan, invoked by former governor Ronald Reagan during the 1974 Arab oil embargo, would allow automobile owners whose license plates end in even numbers to buy gas one day and those with odd numbers to buy gas the next.

No motorist could obtain more than 20 gallons, and no motorist could buy gas if his tank is more than half full.

Emergency, public transportation, commercial and postal vehicles and motorcycles and cars for the handicapped would be exempted.

"It is a modest proposal; It is not going to create new supplies of gasoline," Brown said. California is the first major state to rely on such a plan since the 1973-74 Arab oil embargo.

Brown only briefly mentioned the plan during a hearing here today.

He then turned over the task of giving the details to Richard Maullin, chairman of the State Energy Conservation and Development Commission, and former astronaut Russell Schweickart, his science and technology assistant.

The plan would be implemented on a county-by-county basis under the terms of long-standing state emergency rules. At the request of the boards of supervisors of California's hardest-hit counties, Brown would call an emergency and the counties would implement the plan.

Violators would be subject up to a $500 fine and six months in jail.

Maullin and Schweickart were asked by reporters if the timing of Brown's announcement had anything to do with President Carter's visit to California today. Brown is expected to be a candidate for president against Carter in 1980.

That drew a brusque "no" from the two men.

The first declaration, Schweickart said, probably would include 5 to 10 of California's most populous counties and about 20 cities. Hardest hit today are San Francisco and Santa Clara County, which contains the sprawling metropolis of San Jose.

"One reason for the long lines is the perception that gas is going to run out," Maullin said. "That tends to freak people out, and they get in line and buy as soon as they can."

In Sacramento on Monday, an out-of-gas motorist was shot and critically wounded by another customer who sprayed a gas station with gunfire. On Tuesday in San Francisco, after two cars had jockeyed for position in a gas line, one driver was stabbed and seriously injured.

On Thursday in Los Angeles, a gas station attendant was treated by paramedics after he was gashed on the head with a beer bottle.

The situation has also been tense at southern California stations. Reports of fist fights have been common and the California Highway Patrol declared a "SigAlert" - their code for the worst type of traffic jam - after 400 cars clustered around a single open station in West Hollywood.

A survey of 349 southern California gas stations by the Automobile Club of Southern California showed that an anticipated increase in first-of-the-month gas supplies had failed to occur. As a result, many station owners expect to run out of gas entirely by the May 28 Memorial Day holiday weekend.

Prices, already five cents above the national average, continue to jump wildly in the face of the shortage. The U.S. Department of Energy ordered a service station chain known as Short stop to roll back prices at its 138 stations, which were charging 99.9 cents a gallon for regular.

The most imaginative solution to the shortage came from Exxon station owner Ernie Sanchez in Sunnyvale, a distant San Francisco suburb. Sanchez decreed that the would sell gas only by appointments made the day before and only to Sunnyvale motorists who showed proof of residence.

"I've had too many transients since this gas thing started," Sanchez said. "I want to keep my gas in Sunnyvale where it belongs."

Stephanie Bradfield, a spokesman for the state energy commission, ascribed California's gasoline problems to news reports, which she characterized as oversimplied, predicting 20 percent gasoline cutbacks. She claims the reduction will be much lower, but that panic buying has now ensued.

She also acknowledged that the problem is more severe in California than elsewhere because, "People are not wedded to their cars as much back East."

But, she added, "Gas is in short supply everywhere." CAPTION: Picture 1, GOV. EDMUND G. BROWN Jr. . . . "a modest proposal"; Picture 2, Motorist Skip Goucher soaks in sun while waiting for gas station to reopen in Santa Monica, Calif. UPI