Opponents of gun registration scored a major victory today when the Maryland Senate amended a tough handgun proposal, supported by Gov. Harry Hughes, to allow "self-defense" as a reason for carrying a gun.

The amendment was sponsored by Sen. Norman R. Stone Jr. (D-Baltimore County), who said it was prompted by action last month of Baltimore cabbie Joseph Word Sr., who killed two would-be robbers in his taxi with an unregistered handgun.

"It would be unfortunate to send a man such as that to jail," Stone said.

A majority of senators agreed, approving the amendment 29 to 17 over the objections of Sen. Joseph Curran (D-Baltimore) and Senate President James Clark (D-Howard-Montgomery).

Curran, sponsor of the main bill, argued that Stone's amendment "will create a terrible loophole for the wrong kind of person."

The original legislation, as supported by Hughes and passed by the Senate Judicial Proceedings Committee, called for one year in jail for anyone caught carrying an unregistered, loaded handgun in public.

Even before today's amendment was offered, the sponsors, bowing to concerns that the proposal might result in the arrest of law-abiding citizens such as the cab driver, agreed to allow judges to impose a lighter sentence if they explained in writing the reason for their decision.

But when the bill reached the Senate floor today, gun-control opponents, siding with frightened citizens who reportedly are arming themselves in increasing numbers, used a series of rapid-fire amendments to weaken the legislation.

First they amended the bill to permit the judges to explain their rulings from the bench, without justifying the decisions in writing.

Then the opponents adopted the "self-defense" loophole, which Curran said "really liberalizes the present law." Curran worried that the bill, as amended, may have the effect of encouraging people to buy guns to protect themselves. "It allows anyone who was carrying a handgun without a permit to claim self-defense. That's really dangerous."

Curran said he will attempt to stave off any similar attempts to amend the bill in the House, where it is still in committee.

"As a public policy," Curran said, "we either want to try to discourage people from carrying guns, or the other alternative--which I find more frightening--we encourage people to carry guns." The self-defense loophole, he said, would return Maryland "to cowboy actor Tom Mix and the frontier days."

"Then you'd rather have a man be shot without protecting himself," cried Sen. Frederick C. Malkus (D-Dorchester), who said he was almost killed by two muggers in 1963. "If I had a gun then," he said, "one of them robbers --we'd know who he was today."

Sen. Victor L. Crawford (D-Montgomery) said he believed the amendment passed because most senators recognized that many of their own normally law-abiding constituents are buying and carrying guns. Crawford, a criminal lawyer, recounted the daytime robbery and slaying of the manager of Bish Thompson's seafood restaurant in Bethesda, in which the manager, while falling, was able to shoot his attacker.

"Without this amendment, if the manager had lived, he would have been prosecuted," Crawford said.