Tens of thousands of poor peasants now are plodding through fields of large green bushes in Central America, carefully plucking the small, fragile berries that contain the region's most valuable product: coffee beans.

Coffee is known as Central America's "gold" because of the fortunes it has created and because of the beans' yellow color before roasting. The region's tropical climate, volcanic soil and mountainous terrain are ideal for growing coffee, which flourishes in the changing temperatures of the slopes.

But these same mountains also are home for guerrillas battling central governments in two countries, and the rebel forces have made the coffee harvest one of their top targets. Both El Salvador's Marxist insurgents and Nicaragua's anti-Marxist ones repeatedly have raided coffee farms, burned machinery, and sought to frighten farm owners into abandoning their land.

Attacks in coffee-growing regions of the two countries are expected to cause millions of dollars of damage and to be the focus of fighting through the end of the harvest in early March. Under the protection of well-armed regular troops, however, the bulk of the crop is likely to be gathered in both countries. The governments also should benefit from favorable weather that has increased this year's potential harvest by more than 25 percent over the previous season.

The political distance separating El Salvador's Farabundo Marti National Liberation Front and the Nicaraguan Democratic Force is reflected in their contrasting relationships with the United States: Washington is pouring tens of millions of dollars into efforts to defeat the Salvadoran insurgency, while it funded the Nicaraguan guerrillas until May via the CIA. The Reagan administration plans to ask Congress this spring to resume funding for the Nicaraguan rebels.

But similar circumstances of economics and geography have led both insurgent forces to seek to weaken their countries' respective governments by striking at the nations' main cash crop. Also, the rebels' mountain strongholds are much closer to the coffee fields than to the sugar and cotton plantations in the coastal lowlands where government troops travel more easily.

The coffee harvest began in late November, almost midway through the dry season that Central Americans call summer. The dollars earned from selling coffee to the United States and Western Europe, the two principal markets, are used to import the manufactured products.

"This country has two months to guarantee its foreign exchange for the entire year," said Miguel Barrios, a senior Nicaraguan government agriculture official.

While firm figures are not available, projections indicate that Nicaragua could earn $200 million from coffee exports this season, while El Salvador could bring in $250 million.

The biggest difference between the two guerrilla forces' campaigns against the coffee harvest appears to be in their policies toward pickers: the Salvadoran guerrillas seek to push up wages by threatening to attack farms where workers allegedly are underpaid, while Nicaragua's insurgents try to frighten pickers away from the fields and thus reduce the harvest.

For the third year in a row, the Salvadoran guerrillas' clandestine Radio Venceremos has threatened to raid farms where defined "minimum" wages and working conditions are not met. It said that pickers must receive $1.33 for each can or tray of beans; an average picker can fill six or seven cans in a day.

Since most farms pay only about $1 for a can or tray, the guerrillas have an excuse to attack virtually any farm that they wish. They are not strong enough militarily to enforce their demands on a large scale, however, and the "wage table" appears to be primarily a propaganda tool.

Pickers in Nicaragua are paid about 75 cents per tray. The rate was doubled to that level last year in an effort to attract more workers.

The Nicaraguan guerrillas, seeking to aggravate the shortage of pickers, ambushes trucks carrying government employes or progovernment activists to the fields after easily overwhelming the handful of militia guarding the vehicles.

Neither guerrilla group publicly favors killing civilians in their campaigns against the coffee harvest, but in both countries defenseless persons have been victims. Salvadoran guerrillas killed four young men on Nov. 23, when the youths stumbled across a group of rebels on their way to raid a large coffee processing mill in this town 36 miles northwest of San Salvador.

Nine days earlier, the Nicaraguan insurgents had killed two infants, aged 5 months and 13 months, in an attack on a state-owned coffee farm in the village of La Sorpresa in northern Jinotega province.

The Nicaraguan insurgents, in particular, appear to have a policy of killing government officials or sympathizers regardless of whether they are armed. This is interpreted in Nicaragua as being in line with recommendations of a CIA-written manual that called for guerrillas to "neutralize" government militants -- and that thereby triggered an extended debate in Washington.

On Dec. 13, rebels killed three Nicaraguan government agriculture officials, one education official and five militiamen in an ambush of a truck sent to check the coffee harvest in northern Jinotega province. The truck had driven several miles into an area where the guerrillas were known to be strong, and some observers questioned why the officials had traveled there without a sizable Army escort.

"There aren't enough troops to send with every truck," said Ramon Blandon, the local head of propaganda for the Sandinista government. Officials apparently would rather risk becoming "martyrs for the revolution" than acknowledge that the insurgents dominate any area of the country.

In the Salvadoran rebels' raid in this western town, guerrillas used homemade explosives and caused about $500,000 worth of damage to equipment at the large, prosperous Tazumal coffee mill. But the damage was repaired in two weeks and the mill resumed normal operations, employes said. Moreover, the guerrillas spurred widespread resentment among residents by killing the four youths, apparently to prevent them from sounding an alarm.

"We cannot forget this. We cannot erase this from our memory," said Esperanza Gonzalez, 55, mother of three of the youths killed by the guerrillas. Her sons, aged 18 to 25, were returning from the movies when they were killed by the rebels, she said. Radio Venceremos, which boasted that the raid had shut down the Tazumal mill completely, claimed that the four persons killed were Salvadoran Army troops.