In a parting assessment as chairman of the Senate Intelligence Committee, Sen. Birch Bayh (D-Ind.) warned yesterday against growing demands to undo many of the controls and safeguards placed on U.S. intelligence activities in the past five years.

The secrecy necessary for those activities, Bayh suggested, would make abuses all too commonplace if the guidelines that have been hammered out within the executive branch and the elaborate rules for congressional oversight were significantly weakened.

"Secret activities are necessary to protect our freedoms and national well-being," Bayh said in a report to the Senate. But he added that "secret power should not be used in ways that curtail those freedoms."

Defeated in his bid for reelection after 18 years in the Senate, Bayh voiced his apprehensions about proposals that are likely to gather fresh momentum with the onset of a Republican administration and a Republican majority in the Senate.

The Indiana senator said that he saw no need for a new organizational structure for the intelligence community and maintained that any determined effort to create one would simply distract senior officials from the more urgent task of improving intelligence operations and analysis.

A proposal introduced by several Republican senators last year after consultation with Reagan advisers would split the CIA in two and create a separate agency for covert actions.

Bayh said that "the best way to improve the CIA, as a practical matter, is by strengthening the quality of its efforts rather than forcing it to undergo another period of turmoil."

The conservative Heritage Foundation, in a recent report, has suggested revoking guidelines that had been laid down during the Ford administration to keep counterintelligence activities within the limits of the Constitution and protect the right to privacy.

Alluding to such proposals as "one of the most disturbing recent developments," Bayh pointed out that the existing guidelines "were developed largely by the intelligence community itself, to ensure clear standards for their activities."

He noted the guidelines have won the support of officials such as FBI Director William Webster, who said last June that the FBI has "found these controls entirely workable."

Bayh voiced alarm over calls to repeal the Foreign Intelligence Surveillance Act of 1978, which established a special court to issue warrants, under statutory standards, for electronic surveillance within the United States in the national security field.