After years of heavy defaults that mounted to more than $5 million, Washington's city government has started filing lawsuits against former college students who have failed to repay low-interest student loans.

The first 25 suits, seeking a total of $91.742, were filed last week in D.C. Superior Court. Among those sued, officials said, were a dentist, an airline steward, a lawyer and a Treasury Department program analyst.

Assistant Corporation Counsel Cary D. Pollak said the city plans to file suit against 15 more deliquent borrowers today, claiming an additional $79,156.

Over the next few months, Pollak said, he expects several hundred suits to be filed in an effort to get money from about 2,200 former students who have defaulted on loans, and have made no effort to repay despite a series of letters asking them to do so.

"The suits will show the student community that the District government intends to enforce these loan [obligations]," said Wallace Gee, director of the city government's student loan office. "We decided we had to do this because of the very slow response to all of the letters we've been sending."

"We're going to be pushing [these suits] out like a machine," Pollak said. "We're through messing around."

From 1967 to 1976, nine D.C. banks disbursed about $14.2 million in low-interest student loans that were guaranteed by the city government. About 7,800 Washington residents received the money.

In June 1976, the banks stopped making new loans when the default rate reached 33 percent - about triple the average for all student borrowers nationwide - and the city government ran out of money to honor its guarantee agreement.

Last spring, after a lawsuit by the banks, the federal government agreed to pay 80 percent of the defaulted loans and the city government to other 20 percent. The banks started making loans again with direct federal guarantees. But Gee said the city still was faced with $5.7 million in old loan defaults from about 3,000 delinquent borrowers.

Over the past 18 months, Gee said, his office has persuaded 846 defaulters to sign agreements to repay after pursuing them through letters and by visits from investigators. "We decided to go slowly," Gee said, and try to give every opportunity for the student borrowers to make an agreement with us. We don't want to ruin anybody's career . . . But now we decided we need a another way to show the student community that we really mean business."

In Maryland and Virginia state student loan guarantee agencies have used lawsuits to collect from recalcitrant debtors for more than a decade.

In Virginia, where the default rate is only 2.3 percent compared to 42 percent now in the District, Jane Chittom, director of the state education assistance authority, said the suits are needed "for the overall health of the program. . . . We want people to understand that if they don't pay their Virginia guaranteed loan, something is going to happen to them. Once they know that, most of them pay."

In Maryland the default rate is 9 percent, which is about the average for state-operated loan programs throughout the country. The default rate for the direct federally guaranteed loan program is 12.8 percent.

Leo Kornfield, the deputy U.S. education commissioner for student financial assistance, said yesterday that U.S. attorneys around the country have filed lawsuits against about 2,500 delinquent borrowers since the Carter administration stepped up loan collection efforts last year.

Previously, Kornfield said, the federal government had filed almost no lawsuits to collect on the loans it guaranteed and some states still do not take legal action.

"We patiently try to get the student to start repayment," Kornfield said. "But we came to the conclusion that students weren't going to respond, we had no recourse except to go to court."

Lawsuits also are sued by several area universities to collect loans they make to students. At Georgetown, financial aid director Richard Black said the university files about 30 suits a year. "Most people settle," he said, "when they get a letter from an attorney."

D.C.'s Gee, said about two-thirds or the defaulters are college dropouts. The others, who graduated, include "a high rate of professionals," he said - doctors, lawyers, teachers and white-collar government employes.

Some 271, he said, work for the D.C. government, including 110 employed by the city school system and 43 who work for the University of the District of Columbia.

Gee said city employes are being treated the same as other defaulters.

But if a court judgment is won against them and they still do not pay, Gee said, they would be subject to disciplinary actions, including possible suspension.

"For a long time the attitude has been that if you have a list of creditors and one of them is the D.C. government, then you can put D.C. at the bottom of the list," counsel Pollak remarked. "That's going to change now . . . I think we're past the point of listening to hard luck stories. Now they're going to have to tell it to the judge."

Under the old loan program, students could receive up to $1,000 a year for undergraduate work and $1,500 a year for graduate study. Interest was 7 per-cent annually because of a government subsidy, and borrowers did not have to start repaying until nine months after they graduated or dropped out of school.

In the suits filed so far, the amount claimed by the city ranges from slightly more than $1,000 to slightly over $9,000. In one case two brothers are being sued for $13,015.

While most of those sued live in the city, many now have suburban addresses. "Now that they went to college with a D.C. student loan," Pollak remarked, "they can afford to move out to the suburban."