Wall Street guru Henry Kaufman called yesterday on commercial banks to forgive a significant portion of the $300 billion debt owed to them by Third World nations, saying that no other alternative could give those troubled countries the economic assistance they need.

The opinions of Kaufman, chief economist of Salomon Brothers Inc., tend to carry heavy weight in financial markets. His call for debt relief -- a controversial proposal that is strongly opposed by banks and the Reagan administration -- was his strongest statement to date on the subject.

He said debt forgiveness would have to be accompanied by "positive benefits for the lending institutions and with new net lending from official institutions," such as the World Bank. He recommended a 50 percent to 100 percent increase in the lending resources of the World Bank.

"I believe that debt relief must begin now," Kaufman said in a speech to a New York conference on global debt strategies. "If not, it will be virtually impossible to alleviate the massive payments disequilibrium in the world without a major economic contraction. No economic expansion that is strong enough to benefit developing countries is in the offing."

He also said that debt relief was needed because banks in recent years have not made sizable new loans to debtor nations, even though some $21 billion in new lending is a key element of the debt-management plan of Treasury Secretary James A. Baker III.

Citicorp said yesterday that the multibank package of $1.95 billion in new loans to Argentina agreed to earlier this year is 96 percent subscribed. The Argentine loan agreement includes a number of innovative elements intended to lure banks to sign up early, such as a slightly lower interest rate for banks that commit their funds by a deadline.

A Treasury Department spokesman said the success of the package meant that "commercial banks remain prepared to provide new financing for performing debtors, despite recent concerns raised by the decision by many banks to reserve against their LDC loans . . . . The announcement that the banks have committed about 96 percent of the new money package represents one of the highest percentages ever achieved for any debtor nation in such a short period of time."

Kaufman also endorsed experimental methods of dealing with debt, such as swapping it for equity ownership in debtor countries. But he said it was unlikely to provide much more than "marginal" relief to those nations, which already are suffering from slow worldwide economic growth.

"The classic way out of our present problem would be for the market to restore equilibrium through a severe business contraction," Kaufman said. "This time around, the costs to all players would be immense."

The Reagan administration and commercial banks have opposed debt forgiveness on the grounds that large amounts of principal would have to be forgiven in order to achieve significant reductions in the size of payments. And banks, officials say, would be even more reluctant to come up with new funds if they declare the old loans worth less than their face value.

Kaufman pointed out in response that banks had vastly increased their reserves for possible loan losses, suffering earnings declines and losses for the second quarter as a result, without major declines in the value of their stock prices. Debt relief also should be accompanied by policy reforms that allow the loans to be written off over longer periods, he said.