A FESTIVAL OF NATIONS : At U.N. Birthday Bash, Dignitaries Dine on a Diplomatic Smorgasbord : Celebration: World leaders have their plates full with five-minute speeches, private meetings and press encounters.

The 50th anniversary celebration of the United Nations developed in its second day Monday into a grand diplomatic smorgasbord as the world’s leaders skipped from speeches in the General Assembly to private meetings with their cohorts, encounters with the press and special New York City events.

The presidents and prime ministers, kings and lesser lights chose from a bewildering array of possible activities.

President Jacques Chirac of France, for example, addressed the General Assembly on Monday morning--taking the opportunity to scold the United States for its burgeoning U.N. debt--lunched with a small group of American journalists and then rushed off to a Park Avenue office building for a private meeting with President Jiang Zemin of China.

Cuban President Fidel Castro, who has titillated New York since arriving Saturday, also met with the Chinese president, conferred with scholars at the Council on Foreign Relations and prepared to meet a group of Puerto Rican businessmen at Jimmy’s Cafe near Yankee Stadium.

Another conference celebrity, Yasser Arafat, the chairman of the Palestine Liberation Organization, spoke to the National Jewish Community Relations Advisory Council in an event that would have seemed impossible only a couple of years ago.

Similarly, Argentine President Carlos Menem met with British Prime Minister John Major--the first meeting between leaders of the two countries since the 1982 war over the Falkland Islands.

The speeches at the General Assembly are the main official work of the world leaders. All 185 members of the United Nations plus a dozen or so observers have the right to speak, and almost all are expected to do so before the summit ends tonight, 50 years to the day since ratification of the U.N. Charter.

They are supposed to limit their speeches--71 are scheduled for this afternoon and evening--to five minutes each, and U.N. officials reported that half of the leaders had kept within the limit so far.

France’s Chirac touched two of the favorite themes of the summit in his short speech. He proposed strengthening the U.N. role in economic development by creating a new position of undersecretary general to monitor the various U.N. development agencies. And he berated the organization’s leading debtor, the United States, for pushing the organization toward financial ruin.

“It is not acceptable,” Chirac asserted, “that many countries, including the foremost among them, should let their arrears pile up, thereby leading to bankruptcy an organization to which all the world’s heads of state and government have come, in an unprecedented event, to affirm that it is irreplaceable on this, its anniversary day.”

President Nelson Mandela of South Africa also rang a popular theme by calling for restructuring the Security Council, the United Nations’ decision-making body, so that it would reflect the growing strength and importance of the developing world. The Security Council is now dominated by its five permanent members: the United States, Britain, France, Russia and China.

“The agenda of the next century and the program of action to promote it can only be true to the purposes of this organization if they are set by all of us,” Mandela said.

In a much different vein, President Yoweri Museveni of Uganda blamed colonialism and foreign meddling for the recent outbreak of massacres in Africa. In the tribal wars of pre-colonial days, Museveni insisted, “we never had women, children or captives being killed.”

Instead, he said, “women would be married by the conquerors, children would be adopted, and male captives would be put to work in the fields.”

With a worldwide press corps on hand for the summit, some leaders took the opportunity to stage press conferences, often to defend some of their policies at home.

President Eduardo Frei of Chile, buoyed by news from home that the former leader of the secret police had finally begun to serve a jail sentence in a case that reflected the repression of the former military dictatorship, told a couple of reporters that this proved that “the rule of law prevails and is respected” by all institutions in society.

Bilateral meetings--the scores of sessions between pairs of leaders--took much of everyone’s time. While the spotlight was on President Clinton’s summits with Russian President Boris N. Yeltsin in Hyde Park Monday and with China’s Jiang at Lincoln Center for the Performing Arts today, scores of other such sessions are being held.

The war in Bosnia-Herzegovina dominated more than Monday’s meeting between Clinton and Yeltsin. According to French officials, Chirac warned Clinton in their meeting Sunday that it would be a mistake to weaken Yeltsin, who has insisted that Russian peacekeepers not be subjected to NATO command.

Chirac proposed that the United States and the North Atlantic Treaty Organization allow the Russians to command a sector of Bosnia during the implementation of a peace agreement. Under such a scenario, the officials said, the Russians would not be under the command of NATO generals.