The immigration law that created an amnesty program for illegal immigrants and a system of sanctions for employers has been a "greater success than Congress ever thought it would be," according to a report issued yesterday by Rep. Charles E. Schumer (D-N.Y.).

The 13-page report was prepared by Schumer's staff during the last two weeks and is based on interviews with dozens of immigrant organizations, employer groups, social service agencies and immigration officials.

The report is Schumer's attempt to assess the success of the one-year amnesty program at its mid-way point and the effect of sanctions on employers, and is the only congressional report on the impact of the new law.

"Immigration reform was a riverboat gamble that seems to have worked," said Schumer, a strong proponent of the legislation, noting there are problems in the amnesty program that must be overcome. Schumer was instrumental in drafting a compromise between growers and labor leaders that allowed the legislation to be approved.

But while Schumer was praising the year-old immigration law, which spawned the amnesty program, more than a dozen immigrant rights groups and several congressmen at a separate news conference yesterday decried the negative effects of the law on thousands of illegal Central Americans who do not qualify for amnesty because they arrived in this country after the Jan. 1, 1981, cut-off.

The groups signed a letter they sent to U.S. senators supporting proposed legislation to temporarily halt the deportation of Nicaraguans and Salvadorans while the federal government investigates human rights violations in Nicaragua and El Salvador. In June, the House approved the measure but it is pending in the Senate.

Roberto Alfaro, president of the Central American Refugee Network, which represents 38 immigrant organizations, said there are between 250,000 and 1 million illegal Central Americans in the United States and between 50 and 60 percent of them do not qualify for amnesty. As a result, he said, hundreds have been fired by employers who fear the sanctions.

Alfaro said his organization "repudiates {the immigration law} because we view it as antilabor, antipeople of Latin descent and antifamily. We also repudiate it because it fails to differentiate between immigrants and political refugees."

Maria Elena Borrego, director of The Family Place, a private D.C. social service agency for immigrants, said her center has 60 cases of families in which one spouse or both have been fired because they are not eligible to work in this country. "Momentarily we do not have the capacity to help them all . . . and all we can offer them are temporary short-term solutions," she said.

In his report, Schumer detailed some of the problems plaguing the amnesty program. "The numbers are still lower than we thought, especially in the northeast, and the processing {of the applications} by the {Immigration and Naturalization Service} has been much slower than hoped," he said.

According to the report, "The long wait on applications has also contributed to an environment of uncertainty and fear for the applicants. The longer it takes for applicants to receive resident status, the more doubt and distrust it creates in the minds of aliens who have not yet applied for legalization."