The world's developing countries called on international bankers today to keep credit and cash flowing freely and not to force them onto the shoals of bankruptcy.

But the chief banking countries, on the eve here of an International Monetary Fund policy board meeting, told the poor nations that their plight, while serious, isn't catastrophic and signaled that they are putting off for the time being any major reform of the world's monetary system.

Against the backdrop of a generally grim world economic outlook, Third World financial ministers outlined a particular bleak scene for themselves, marked by rising oil bills on the one hand and on the other by a weakening of the export markets that otherwise would provide the cash to finance the oil.

Able in the past to take advantage of world credit markets swollen with Organization of Petroleum Exporting Countries deposits that banks were eager to lend, the developing countries noted with alarm that leading central banks are tightening lending requirements as part of the industrialized world's joint efforts to squeeze credit and choke inflation. This has tended to aggravate already strained relations between the rich and poor nations.

In a communique issued today by the so-called Group of 24-finance ministers representing the developing countries, the poor nations lodged a strong appeal for more credit on softer terms.

"Since the role of the private financial system has become increasingly important and essential for many less developed countries, it is fundamental that this market should be kept free from interference," the statement said.

The major banking countries took issue with the suggestion that they are interfering unduly with the world's free lending markets.

"What has been done would not constitute undue restrictions but prudent measures we are obliged to take," said West Germany's state sectretary for finance, Manfred Lahnstein, referring to restrictive measures adopted at a recent meeting of central bankers in Basel, Switzerland.

While voicing sympathy with developing countries' needs, financial officials of the rich countries appeared to hold a rather different assessment of those needs.

They argued that although international payments imbalances have grown as a result of last December's oil price jumps, th deficits remain manageable under the world's current financing arrangements. Moreover, the International Monetary Fund, with as much as $25 billion to lend, was said to be sufficiently liquid at present to help finance an aggregate international payments deficit this year of $120 billion.

The basic list of the poor countries' demands is familiar. It includes the continued sale of IMF gold holdings (now down to 100 million ounces worth about $50 billion) to benefit them, expansion of their participation in the fund, increase in special drawing rights, lighter conditions for borrowing money and longer-term loans for general purposes rather than specific projects.

The poor countries had been hoping to gain some leverage over the rich by witholding approval of a proposed dollar substitution account favored by the United States and by European and OPEC nations to mop up surplus dollars and provide greater stability to the world monetary system.

Under this plan, the world's central banks would put their dollars on deposits with the IMF in exchange for special drawing rights, which are a sort of artificial money based on the weighted value of other international currencies.

Despite indications at last autumn's IMF meeting that the substitution account would be approved here this week, U.S. officials made it publicly clear before arriving that American wasn't ready to press for acceptance at this time. Without American backing to solve still difficult political and technical problems with the proposed account, the idea was tabled.

"Everybody welcomes the account," said Lahnstein. But West german Finance Minister Hans Matthoefer, host for the meeting, told reporters last night that he doesn't expect a decision on the matter -- or even an agreement in principle -- this year.