More than 100 members of Congress have written President Reagan opposing U.S. assistance to rebels fighting the Marxist government in Angola.

The letter states that U.S. involvement in the Angolan conflict, "whether direct or indirect, covert or overt, would damage our relations with governments throughout Africa and undermine fundamental U.S. policy objectives in southern Africa."

Dated last Friday, it was initiated by Rep. Matthew F. McHugh (D-N.Y.), a member of the House Permanent Select Committee on Intelligence.

Reagan announced that day that he favors a covert-aid operation to the rebel group, the National Union for the Total Independence of Angola (UNITA). He said it would be "more useful" and "have more chance of success" than various bills in Congress calling for either open military or humanitarian assistance.

More than 100 members of Congress are cosponsoring a bill that was introduced by Rep. Mark D. Siljander (R-Mich.) and authorizes $27 million in overt military aid to UNITA.

Another bill, sponsored by House Rules Committee Chairman Claude Pepper (D-Fla.) and Rep. Jack Kemp (R-N.Y.), would provide the same amount in humanitarian assistance.

Supporters and opponents of U.S. aid to UNITA have been mobilizing forces in and outside of Congress in recent weeks as the administration has continued to debate the controversial shift in U.S. policy toward the Angolan conflict.

Despite Reagan's open backing of covert aid to UNITA, the administration has not sent a formal presidential finding of such a decision to Congress' two intelligence oversight committees, according to congressional sources.

A White House spokesman said yesterday that no decision has been made on such a covert-aid program.

While the panels could not block a covert-aid plan by the Central Intelligence Agency, they can express their views on any such operation and could, if opposed, eventually act to cut off funding.

Among those recently expressing opposition to the administration plans is the Organization of Africa Unity (OAU).

Its secretary general, Ide Oumarou, sent letters this month to Reagan, House Speaker Thomas P. (Tip) O'Neill Jr. (D-Mass.) and Senate Majority Leader Robert J. Dole (R-Kan.) saying U.S. aid to UNITA would constitute "a gross violation of Angolan sovereignty and territorial integrity" and impede U.S.-led negotiations aimed at obtaining independence from South Africa for Namibia.

A group of African ambassadors met Nov. 8 with Vernon A. Walters, U.S. ambassador to the United Nations, to express opposition to U.S. aid for UNITA. They also presented him with an OAU memorandum warning the United States that such aid would be considered "a hostile act against" the OAU.

Among influential Americans opposing U.S. aid to UNITA is David Rockefeller, former chairman of Chase Manhattan Bank.

In a letter Nov. 12 to Rep. Howard E. Wolpe (D-Mich.), chairman of the House Foreign Affairs subcommittee on Africa, Rockefeller said such U.S. involvement with UNITA would mark a sharp departure from administration policy, creating "intolerable risks" that would "needlessly endanger American lives and American property" in Angola.

Rockefeller said the U.S. Export-Import Bank, with approval of the National Security Council, has been encouraging expansion of trade between Angola and the United States that now totals $1 billion and of U.S. investment there, now more than $500 million. "Angola has become one of the largest and best economic partners" of the United States in Africa, he said.