The Reagan administration says proposed legislation to extend constitutional protections to hundreds of Cuban refugees being held by American immigration authorities would only delay the process of paroling eligible detainees.

Two separate bills, drafted after the riots last fall by Cuban detainees in Atlanta and Oakdale, La., would bar the government from indefinitely detaining, without a court hearing, aliens who legally are "excludable" under immigration law but who cannot be deported.The Immigration and Naturalization Service had found many of the Cubans who arrived in this country during the Mariel boatlift in 1980 to be unacceptable for admission to the United States, but Cuban President Fidel Castro refused to take them back.

Those excludable aliens consisted of mental patients and people who had committed crimes in Cuba or since their arrival in the United States.

The riots broke out at prisons at Atlanta and Oakdale after the United States tried to reinstate a 1984 agreement calling for the deportation of 2,746 such Mariel Cubans.

Deputy Associate Attorney General Cary H. Copeland, testifying Wednesday before a House panel, criticized one bill as too costly and said the other could force the government to release dangerous aliens into American society.

"We believe we have a system that can work and will work, and we hope you will give it an opportunity to work," Copeland told the House Judiciary subcommittee on refugees and immigration law.

One bill, sponsored by Rep. Robert Kastenmeier, D-Wis., would grant any Cuban detainee denied parole by both INS and Justice Department review boards another hearing within 90 days before an administrative law judge.

At that hearing, the detainee would be represented by an attorney paid for by the government and would be able to call witnesses and present evidence in his behalf and question witnesses against him.

The other bill, sponsored by Rep. Pat Swindall, R-Ga., would limit to six months the amount of time a person could be imprisoned by the government as an "excludable alien." After six months, the person would be reclassified as a "deportable alien" and thus would be entitled to constitutional protections similar to those outlined in Kastenmeier's bill.

Copeland said the government cannot afford to provide attorneys to the Cuban detainees and doesn't have enough administrative law judges to hear their appeals. He said Swindall's bill would "essentially mandate that Mariel Cubans who were not ready for parole due to violent propensities be released."