The uneasy peace of an armed camp has descended on the lush Imperial Valley, where both sides are bracing for new violence in the costly month-old lettuce strike.

Heavily armed police and sheriff's deputies today surrounded the fields where a 27-year-old farmworker was shot to death a week ago. A hundred pickets, their number limited by a new injunction, watched silently as a handful of would-be strikebreakers cut and loaded over-ripe lettuce under a warm desert sun.

But Cesar Chavez, who had vowed to return to the field where Rufino Contreras was murdered, instead was 225 miles away in Los Angeles, meeting with top officials of one of the nation's largest lettuce growers.

According to accounts on both sides, Chavez had decided to avoid risking further violence, and is instead using the unintended martyrdom of Contreras to crack the fragile unity of the 10 struck growers. One growers' spokesman said the killing was "a boost for the union and a blow for us."

While union officials were careful to express continued sorrow over the murder of Contreras, who left a convalescing wife and two small children, their political assessment of what had happened in the strike was identical to that of the growers.

"The union members knew all along they had a goal," said United Farm Workers aide Marc Grossman. "Now they have a martyr."

Chavez' meetings are with officials of Sun Harvest Inc., a Salinas-based grower that cultivates about 3,000 acres of lettuce and other produce in the Imperial Valley.

Sun Harvest is the largest of the struck growers, and is a particularly vulnerable target because it is a subsidiary of the Boston-based United Brands Co., which has many products (Chiquita bananas, Morrell meats) that could be damaged by a consumer boycott. It is in past boycotts of crops such as grapes and lettuce that the UFW has proved especially effective.

Even if Sun Harvest breaks ranks, however, several growers here say they will continue to resist the UFW call for a 180 percent wage increase that the smaller growers say eventually will force them out of business. Neither the union nor the growers, who are offering a 7 percent increase of 26 cents in the basic hourly wage, has budged off its original position.

The lack of any real negotiations between the two sides has created a sense of frustration bordering on desperation here in the Imperial Valley, where only a week or two remains in the planting season for valuable canteloupe and wheat crops.

"This whole thing makes no sense for anyone," says lettuce harvester Tom Hubbard. "The fields will not be back to normal for months. If they would plant now, they'd at least have a crop to strike in the summer."

Hubbard is typical of the high-risk entrepreneurs who gamble in the risky crop of lettuce.Technically not a grower, he has contracted to harvest 1,200 acres of lettuce and take the proceeds of half the acreage as his payment.

"It's a dice roll," Hubbard says. "Lettuce-growing is free enterprise in its finest form."

Since Hubbard's workers have not struck, he has been making money hand-over-hoe in the current harvest. Lettuce prices have soared from $5 a 24-head carton at the beginning of the season to as high as $12 a carton. This week the prevailing price in the Imperial Valley is $10 a carton.

Unlike many other major crops, there is very little artificial regulation of lettuce supply. Prices are determined by the market, and swing wildly from week to week. Th original high price of lettuce this year was caused not by the strike, but by severe winter rain that created expectation of a scarce winter lettuce crop. This was followed by a budworm scare, which also boosted prices.

The strike has accentuated the roulette wheel quality of lettuce-growing. Some of the struck growers are talking glumly about going out of business, while others are making as much as $6,000 an acre from their lettuce crop.

The same kind of disparity exists for farm workers.On the non-struck farms (18 of 28 in the Imperial Valley have union contracts), workers on picec-rates are regularly making $450 a week. Their fellow UFW members on strike are receiving a $25 weekly union benefit.

It is not lettuce alone that will be affected by the current dispute. The prediction on both sides in the Imperial Valley is that the strike will spread to other states and other crops as the migrant harvest pattern continues.

If the strikes spread widely, they could also set the pattern for renewed agricultural labor violence. Scarcely a day passes in the present hostile atmosphere where there is not some scattered outbreak of violence somewhere in the state, such as occurred Thursday in Oxnard, 300 miles northwest of here, when rocks were thrown at a busload of workers going to a Sun Harvest celery farm.

The violence also has racial undertones. In El Centro this week one of the social events is a lettuce harvest ball, sponsored by Los Vigilantes, a group whose name brings memories of the anti-Mexican violence once common in Imperial County.

When Contreras was shot to death, he and a group of strikers were attempting to talk to a group of Filipino "replacement workers" who since have left the area.

Three men, including the foreman of the Saikhon Ranch, where the killing occurred, are scheduled to be arraigned in the killing next week.

Sheriff's deputies here say that tensions continue high on both sides. They were obviously relived when Chavez decided to negotiate in Los Angeles rather than make his symbolic confrontation in El Centro. But the decision may only have postponed Chavez' ultimate action.

"He is still determined to go into the fields at the next opportunity," said Grossman.

Among those responsible for protecting the peace here there is a wide-spread hope that the opportunity never comes.