When the new immigration law was signed by President Reagan in November, Manuel Piche believed that his seven-year odyssey of living in this country illegally was almost over because he would be granted amnesty. Now, though, he realizes that he may not qualify because the law says an amnesty applicant must not be a public charge.

Piche, 33, a native of El Salvador who lives in Northeast Washington, was fired from his Bethesda construction job in December and has not been able to find other work because of his illegal status.

He and some other immigrants are trapped between two of the law's provisions: It virtually mandates that applicants for amnesty be employed, but at the same time it contains penalties for employers that have led some to fire undocumented workers.

A recent lawsuit in Houston on behalf of three workers who had been eligible for amnesty and who were fired from their jobs has focused attention on this problem, which some people predict may get worse after the employer sanctions take effect next month.

Duke Austin, a spokesman for the Immigration and Naturalization Service, said the agency has received "scattered reports" of firings and is discouraging employers from dismissing amnesty applicants. He said, for example, that he recently persuaded officials of a crating firm not to fire their undocumented workers.

But lawyers and workers at social service agencies say that, nationwide, they have heard of hundreds of undocumented workers being fired since the immigration law was signed Nov. 6.

Rick Kenney, another INS spokesman, said undocumented workers would have "a hard time" qualifying for amnesty if they did not have a job. Kenney added, however, that "we will give applicants the benefit of the doubt."

The law grants amnesty to undocumented immigrants who have been in this country since Jan. 1, 1982, and it imposes stiff fines on employers who knowingly hire illegal aliens after July 1.

During the first year of employer sanctions, first offenders will simply be given warnings. Thereafter, employers will receive penalties that include civil fines of $250 to $10,000 for each illegal alien hired, and repeat offenders will face criminal penalties of up to six months in prison for each violation.

However, employers cannot be fined for undocumented workers they hired before Nov. 6, and those hired since then have until September to present proof that they are seeking official permission to work in this country, immigration officials said. Employers need only a letter from workers saying that they intend to apply for official status before September to satisfy the INS.

The law has caused confusion among businesses, resulting in unnecessary firings, community leaders say.

One local construction company, Miller and Long of Bethesda, fired 70 undocumented workers in December, including Piche. Myles Gladstone, the company's personnel director, said the workers were fired not because of the immigration law but because they were using false Social Security numbers.

Since then, Gladstone said, the company has not fired any undocumented workers, and it recently included letters with employe paychecks offering to pay the $185 amnesty application fee.

Employers face another issue: the possibility of being charged with discrimination for firing or refusing to hire immigrants who are eligible for amnesty.

Immigration lawyers contend that aliens who are eligible for amnesty are protected by an antidiscrimination provision in the immigration law that says that employers cannot discriminate against "a citizen or intending citizen" because of that person's "citizenship status."

A federal judge in Houston cited that provision recently in ordering that three illegal aliens fired for using false Social Security cards be reinstated because they were eligible for amnesty.

In Los Angeles recently, the Mexican American Legal and Education Defense Fund (Maldef) filed a discrimination complaint against a Sears, Roebuck & Co. store with the federal Equal Employment Opportunity Commission because the store dismissed three undocumented workers who are eligible for amnesty. Store officials announced within hours that they would reinstate the workers.

"When we hear of these firings, what we do is call the employers and explain the law, and usually the workers are reinstated," said Maldef legal director E. Richard Larson.

"If employers discriminate against those who are entitled to work, they are opening themselves up to major litigation," said Wade Henderson, assistant director of the legislative office of the American Civil Liberties Union. "Employers now are sailing in uncharted waters . . . . We advise them to have caution in changing their personnel procedures."

Government lawyers say undocumented workers are not covered by the antidiscrimination protections in the law because it excludes "unauthorized aliens."

"If Congress had wanted to, it could have said an employer can't discriminate against an employe who can apply for amnesty, but unfortunately {Congress} didn't," said Mary Mann, the acting special counsel who was selected by President Reagan in April to oversee the start-up of a special Justice Department office to oversee immigration law discrimination complaints.

Mann said that workers who are unfairly fired can pursue their discrimination complaints through the Equal Employment Opportunity Commission instead. She says her office will handle complaints only from people who are here legally who believe that they have been discriminated against because of national origin or citizenship status.

"The buzzword now among employers is discrimination," said John O'Connor, an official with the Washington Building Trades Council, an organization representing 30,000 local construction workers. "Employers would be hard pressed to tell if an undocumented worker is legal or illegal just because he doesn't speak English."

Meanwhile, local social service workers report seeing the apparent effects of the job squeeze being applied to undocumented workers. The Rev. Kevin Farrell, director of the Spanish Catholic Center in Adams-Morgan, said that this year his agency will help about 45,000 people, compared with about 30,000 last year. He attributes some of the increase to firings.

"What we're getting are mothers with children coming to us looking for food because their husbands can't find work," he said. The center also offers job referrals, free clothing and other services.

Piche, who crossed the Mexican border into the United States in March 1981, is one of those who hang on, sustained by the slim hope that he will receive amnesty someday.

Because he lived with a brother in Los Angeles when he first entered the country and used false names and Social Security numbers in his first two jobs, Piche is unsure whether these documents will suffice to qualify for amnesty.

He now survives day by day in a $700-a-month rented Northwest Washington house he shares with 12 relatives. His wife Maria earns about $150 a week sewing at home.

Every day, he leaves the dingy row house and rides buses to construction sites where he hopes to get a job. He has an appointment this month at a legal aid office to begin the paper work to apply for amnesty.

During the past few months, many of his friends have lost their jobs and returned to El Salvador, he said. But he has brushed aside any thoughts of going back to his native village of Zulutan with Maria and their two children, ages 2 and 7.

He said he witnessed the execution of a boyhood friend and fears he will be killed if he returns. "In El Salvador," he said, "you survive off the mangoes on the trees. Here, at least, we have a roof over our heads and food to eat."