Unable to fulfill austerity plans or meet deadlines for foreign debt payments, Peru has stumbled into a financial crisis that could strangle the country's economy even as its democratic government struggles to finish its term.

Already cut off from funding by the International Monetary Fund and banks, the government of President Fernando Belaunde Terry is now behind by about $300 million in interest payments on the country's $13 billion foreign debt. Banks have reduced trade credits to Peru by more than 60 percent since June, and the Reagan administration so far has declined to step in with special aid.

As a result, Peru now stands closer to financial collapse than any other major Latin American debtor. If the country does not pay its interest or reach new agreements with banks and the IMF, its loans could be declared "value impaired" in March by U.S. bank regulators.

The result would be the suspension of virtually all international credit to Peru, paralyzing its foreign commerce. Experts here said U.S. banks, who could be forced by regulators to increase their reserves to cover possible losses, might also declare the country in default and attempt to attach its assets abroad.

"It's a serious situation," said a bank executive. "The government needs to start acting, and it can't wait much longer."

For Belaunde's government, the debt crunch is particularly dangerous because it comes only four months before crucial national elections for president and Congress. Belaunde, who is barred from reelection, hopes to cap a 30-year political career by becoming the country's first democratic president to turn over power to an elected successor in more than 60 years. Some government officials are concerned that harsh new austerity measures to satisfy creditors, or the threatened alternative of financial collapse, could destabilize the government in its final months.

Belaunde's ministers, who unsuccessfully sought to put together an emergency package of loans from other countries last month, were reported yesterday to have paid $50 million in interest to keep from falling more than six months behind in interest payments on any loan. The Congress also has agreed to some measures, such as increasing gasoline prices, demanded by the IMF as a precondition for negotiations on a new economic package.

However, Peruvian officials say they have not found a way to pay their debts yet and still are unwilling to take some of the measures demanded by the IMF and creditors. "We don't know what the solution is yet," said Manuel Ulloa, a senator and former prime minister who has conducted special negotiations with banks and the IMF this month. "One tries to avoid extreme measures. But if you don't have the money, you don't have it."

In some respects, Peru's difficulties have been increased by the relative easing of debt problems in other major Latin American countries. Mexico, Brazil, Venezuela and Argentina all have managed to reschedule their debts and keep up interest payments, and bankers say the sense of generalized urgency about Latin American loans has dwindled, at least temporarily.

"The pressure's off. No one is talking about the debt bomb, at least in the short term," said the banker. "And in that light, Peru has behaved very badly. A lot of people are disgusted with Peru."

"The banking community can afford to eat Peru. They can take the loss and just cut the country off," warned a diplomat following the situation. "Some bankers think the consequences could be rather salutary."

Government officials, while conceding that Peru has not been able to fulfill its previous agreements with creditors, argue that the country does not deserve such harsh judgments. In the last two years, they point out, Peru has been stricken by a series of natural disasters and widespread violence by the Maoist Shining Path movement, forcing the government to spend far more than it planned.

If spending for disaster relief and the battle against insurgents is discounted, "our budget deficit is within what the IMF asked for," said Prime Minister Luis Percovitch in an interview. "What we ask is that the IMF and banks examine our situation not only mathematically but consider these other factors and take a realistic view of our problems."

Bankers and diplomats here readily concede that Belaunde has faced economic reverses beyond his control throughout the 4 1/2 years of his government. These officials and many Peruvian experts outside the government believe, however, that the president repeatedly has compounded his troubles by refusing to adjust ambitious government programs to the changed economic conditions.

An architect by profession, consumed by visions of roads, bridges and settlements taming Peru's vast frontiers, Belaunde inaugurated his government with plans for $11 billion in government development projects that clearly remain his first priority. Despite the critical government financial problems, the president has appeared more determined than ever to complete prized projects in the waning months of his career and has refused to cut spending on them to meet budget deficit targets.

"Belaunde is a builder, not an economist," said Felipe Ortiz de Zevallos, a former government official and respected economist. "He thinks the job of the economy minister is simply to get him money for his projects."

Early this year, Belaunde personally worked to complete an economic agreement with the IMF that called for a major reduction of government deficit spending, controls on wages and other austerity measures. By June, officials also had reached agreement with banks on the rescheduling of $1.7 billion in loans falling due through July 1985, prompting the president to declare that the country's economic obligations until the end of his government had been resolved.

Peru, however, quickly proved unable to live up to its agreements. Under strong pressure from striking workers, military commanders and the development-intent president, ministries and state companies soon began overspending, pushing the government deficit to twice the IMF target.

In addition, Peru stopped paying interest on its bank loans in June because of lack of government funds. Banks then refused to complete the rescheduling agreement or provide new loans, and the IMF suspended its own loan program with Peru in September.

Bankers and other sources close to the government here said that Belaunde, an ally of U.S. governments since the Kennedy administration, believes that the United States ultimately will step in and bail out his administration. U.S. officials say, however, that the Reagan administration currently is not willing to come to Peru's aid.

As a result, many experts here believe that the financial crisis may be managed with stopgap measures and ultimately passed on to the new government scheduled to take office in late July. In that case, the new president will have to act almost immediately to avoid a crisis.

"The new administration will have to take drastic measures from its first day to salvage the situation," warned Ortiz. "Otherwise, we could be ruined."