Attorney General Griffin B. Bell told the Senate yesterday he believes President Carter had the legal authority to negotiate and sign the Panama Canal treaties without first obtaining the approval of Congress.

In testimony before the Senate Foreign Relations Committee. Bell sought to counter the constitutional argument raised by forces seeking to prevent approval of the treaties by the Senate.

Some treaty foes contend the Constitution gives Congress the sole power to dispose of territory or property belonging to the United States. As a result, they say. Carter acted improperly in negotiating the treaties, which transfer eventual control of the canal to Panama, without first obtaining the legislative permission of both House and Senate.

The administration counters that the Constitution also gives the President the power to enter into treaties with other governments, subject only to approval by two-thirds of the Senate. Therefore, the administration argues, the Constitution provides "concurrent authority" to transfer property either by treaty or by Senate-House action.

That possition was backed by Bell, who testified in his capacity as "the lawyer for the United States." He said the Justice Department's study of applicable Supreme Court decisions has led him to conclude that a treaty the equivalent of a legislative act and that "the mere existence of a congressional power to legislate in the field does not preclude a treaty from being self-executing."

To buttress his argument, Bell noted that several Supreme Court decisions dealing with Indian treaties allowed the disposal of territory without prior legislative authorization.

He also pointed out "that throughout our history presidents, acting with the advice and consent of the Senate, have made numerous self-executing treaties transferring territory of property belonging to or claimed by the United States."

As examples, he cited 19th Century treaties under which the United States gave or exchanged territory with Spain and Britain. In each case, he pointed out, there was no prior authorization by Congress and the treaties were considered to be legally in force after their approval by the Senate.