The largest summit conference in modern history opens here Monday when more than 60 heads of state gather for the sixth meeting of the nonaligned movement.

From the first nonaligned meeting in Belgrade in 1961, attended by 25 nations, the movement has grown to 93 members, including seven countries added this week. Its membership spans most of the southern half of the globe, along with much of the Middle East and southern Asia and even a small part of Europe.

For Cuba, hosting the summit is both a major international victory and an opportunity to consolidate a Third World leadership position that belies its small size and legitimizes its view of geopolitics.

In addition to pressing its views in preliminary summit meetings and in documents distributed last week, Cuba has exerted an extraordinary physical effort and spent more than $140 million to make the summit a success.

In terms of major events, the past year has been the equivalent of the U.S. Bicentennial for Cuba, with massive, internationally attended celebrations for last August's International Youth Festival, the revolution's 20th and a new 247-room hotel, especially for the summit.

In its most intense construction program since huge housing projects went up literally overnight in the revolution's early days, the Cubans have built a $35 million conference center and new 247-room hotel, especially for the summit.

Six other luxury hotesl in Havana have been refurbished. The sprucing up extends to new uniforms for hotel employes and newly printed menus. Miles of new roads have been laid, and miles more repaved.

At least two new sewage plants were built, and nearly 200 decaying mansions were renovated to house the high-level delegations.

For the past year, Cubans have been told that summit preparations were a patriotic duty. Organizations ranging from state enterprises to neighborhood block councils set summit task goals. Preparatory committee vice chairman Ramiro Valdez last week called those organizations' achievements "a legitimate triumph of the revolution and the people."

President Fidel Castro and Vice President Raul Castro showed up last Tuesday to inspect the new "convention palace" summit site. Over the past several days, in constant trips to the airport, Fidel Castro has personally greeted at least 25 heads of state, with many more due to arrive.

The hundreds of accredited journalists here have been housed primarily in the ancient and newly oppulent Hotel Nacional, where carpets were still being laid and a new telephone system was installed last Sunday. While the journalists are allowed in the lobbies of the delegates' hotels, they are prohibited from elevators and hallways without the written permission of the person they want to visit.

The same restrictions apply to diplomats at the U.S. interest section here, posing a difficult problem for the Americans -- who are intensely interested in debates and opportunities to lobby.

"It's hard for us to be seen so publicly with what is denounced here as the world's principal imperialist force," one Asian delegate said.

Yesterday, divisions among the delegations extended out of the closed plenary sessions to the press room. When the Singapore delegation called a press conference to present its side of a hot dispute with Cuba over the seating of the Cambodian delegation, Cuban and Vietnamese journalists packed the front row seats and monopolized the session with aggressive and highly political questions.

Despite the number of Cuban reporters present, however, the press conference received no montion in today's edition of Granma, Cuba's principal newspaper.

The Cubans have overcome the enormous logistical challenges presented by the summit. Cuba suffers from shortages of communications facilities, motor vehicles and food of the kind most visiting government officials are accustomed to.

What appear to be hundreds of new Fiats and Mercedes and buses have been purchased to transport the visitors around town. Scores of chronically scarce taxicabs have been pressed into summit service.

Well-equipped new press rooms at several locations offer instant telex and telephone communications around the world, a welcome surprise for journalists who have waited days to make an overseas call out of Cuba during past visits.

Hotel and other local restaurants have more than adequate supplies of food and liquor, on a scale that Cuba has not seen for 20 years. The massive new conference center restaurant, with sophisticated design and furnishings, offers a wide range of imported items that surely have taken an incalculable cut in the hard currency reserves of Cuba.

Cuban officials say the expenditures will pay off, not only during the summit, but for years to come. Anxious to expand its tourist industry as a foreign exchange earner, Cuba plans to use the conference center and the hotels to attract international conventions and visitors in the future.