As fighting to topple the government of President Anastasio Somoza continues here, thousands of Nicaraguans face the long-term problem of mass starvation and the probable ruin of food and export crops that provide more than half the country's income.

"Even if he wins a temporary military battle, Samoza will never again be able to govern this country," one opposition leader said, "not only because of the hatred he has engendered, but because there won't be anything left to govern."

Since a nationwide strike against Samoza began more than three weeks ago, followed by a violent national mutiny against his National Guard, Nicaragua has moved to the brink of total economic collapse.

Bankers and businessmen here say the economic situation has deteriorated to the point where even an immediate cease-fire would do little to change the grim prognosis.

Meanwhile, National Guard claimed to have retaken the northwestern city of Chinandega from rebels who had held it for nearly a week, while fighting continued in the northern provincial capital in Esteli.

Guerrillas of the Sandinista Liberation Front renewed their attack, launched Sunday from Costa Rica in the south, on the frontier village of Penas Blancas.

The symptons of Nicaragua's economic disease are several and each on its own could be fatal.

First is what central bank president Roberto Incer described in an interview yesterday as a "massive flight" of capital, amounting to $40 million in the past three months, taken out of the country by wealthy Nicaraguans fearing a government overthrow.

"That might not seem like much money to some countries," said one leading commercial banker, "but to a little country like this, it's phenomenal." A similar amount of private capital left the country during a two-week protest strike last February, and reportedly has not returned.

One banker estimated withdrawals at $2 million a day before the central bank instituted emergency restrictions on dollar buying last week.

Airlines flying out of Nicaragua have stopped taking reservations and now tell all who want a ticket to show up at the airport at 6 a.m. and wait on a first-come first-served basis. Packed planeloads of wealthy and middle class families leave the country daily.

Virtually no money is coming into Nicaragua. A $40 million International Monetary Fund standby credit agreement arranged last month has been sheled indefinitely while Nicaragua's crisis continues.

Another $40 million loan the government requested from eight foreign commercial banks was agreed to on the conditions the IMF credit is approved. Only one of the eight participating institutions, the Bank of London has disbursed its $4 million part of the package.

"That money," said a financial analyst here, "was intended to cover government operating expenses until the end of the year." Much of it would have paid the salaries of government employes. Teachers, who went unpaid for most of the summer, found that government checks, finally received late last month, bounced.

Local and foreign commercial banks here have largely ignored the strike - because of government threats of license removal and arrests of striking employes.

While most other enterprises have remained closed, and very few people other than government employes are working, small businesses and stores in at least five of the nation's largest provincial capitals have been completely destroyed by fires and National Guard shelling.

Inventories have been totally wiped out by looters and a larger percentage of Nicaragua's small enterprises are in ruin.

"Somoza is burning up the country," the opposition leader said, citing his refusal to resign under pressure that many Nicaraguans say would have toppled any other latin American government weeks ago, and the military retaliation that has affected civilians more than the guerrillas.

The third, and most devastating aspect of the economic carnage, is in agriculture.

More than half of Nicaragua's yearly revenues come from the export of cotton, coffee and, to a much lesser extent, sugar and beef. Nearly all of the cotton and coffee - Nicaragua's biggest cash crops - is grown in the northern part of the country where the fighting has been heaviest.

When he was asked in a news conference three weeks ago to assess the impact of the strike, Somoza said it would do no lasting economic harm to the country because "the greatest source of work in Nicaragua is agriculture and this has not been affected."

But small farmers and large plantation owners vehemently say Somoza is wrong. Coffee beans are now ripe for picking, but most peasants are either on strike or scared to venture outside their homes in the slum areas around the northern cities of the agriculture belt.

For cotton, the problem is not as immediate, but potentially more disastrous. While cotton is pocked in January and February, it must be carefully tended and sprayed with fertilizer and insecticide beginning in September.

One cotton plantation owner outside Leon said last week that all three of his crop dusting planes were destroyed by National Guard air attacks on the city. At the same time, while the central bank has ordered local fertilizer and insecticide producers and distributors to stay open on threat of having government guaranteed loans immediately called in, this farmer and others said there is no way to spray the crops without workers.

Hundreds of thousands of Nicaraguans are employed as agricultural workers. Most are paid at sunset daily, approximately $1.50 to $2.00 for 12 hours work.

"If you work a day," one peasant woman said, "you ate a day."

Those who are not now tending cotton or picking coffee have no money to buy food, and thousands from around the devastated northern towns are now housed in Red Cross refugee camps.

Even if they had the money, however, food is growing increasingly scarce at any price.

According to central bank president Incer, the february strike caused a 1 percent loss in this year's gross national product, and the current strike will cause a larger drop. After years of growth rates averaging 5 percent, he said, "during 1978 we expect a zero rate of growth."