Africa's first major cultural festival of the decade opens here today with its Nigerian hosts proud - and privately even a bit surprised - that the great day is finally upon them.

The nation's capital, once known as the "Calcutta of Africa" for its crowds, open sewers and filth, has undergone a relative transformation in honor of the 15,000 performers, artisans and academics from 57 countries the government estimates are here for the Second World and African Festival Arts and Culture (FESTAC).

Streetlights on the main Lagos avenues are decorated with the stunning gold-colored copies of original Nigerian bronzes and attractive copies of African cloth patterns. The Gordian knot of the capital's infamous traffic has been cut, and cars now flow easily where they once inched at less than two miles an hour across the only two bridges connecting the island city with the mainland.

The traffic miracle, accomplished by a somewhat Draconian military decree staggering (by license plate number) the days on which individual motorists are allowed on the streets, has even prompted suggestions that the military police put away their three-foot-long braided horsewhips - known as "kobokos" - which they wield against recalcitrant drivers.

Marred by multiple postponements, a civil war, a coup and an attempted coup, mismanagement, extravagant contracts, bribes and inter-African squabbles, the mere fact that the festival is taking place is viewed as an accomplishment.

Many Nigerians are amused by suggestions from the Balkanized, under-populated, often dirt-poor former French colonies in Africa that oil-rich Nigeria, whose population is a fifth of the entire continent's, is using FESTAC as a cultural weapon to spread a claim to Third World leadership.

FESTAC, in fact> was both a contributing cause to the 1975 coup that overthrew Gen. Yakubu Gowon, and a kind of albatross that the new rulers felt they had to honor for Nigeria's prestige.

Under Commander Ochegombic Promise Fingesi, FESTAC was cut back in mid-1975 to mirror the new military rulers' distaste for the proflagate spending associated with Gowon's government, after the end of the Biafran civil war in 1970 insured Lagos' control of the country's oil wealth.

Canceled were such extravagant projects as one foreign airline's plan to supply 30,000 daily tray meals to FESTAC guests and another airline's contract to fly their linen to Europe for laundering.

Under the new military rulers' "clean sweep" brigade, FESTAC refused to honor earlier promises to bankroll some overseas committee if they were unable to raise sufficient funds on their own.

One of the principal victims of complex fiscal woes was the American delegation. It once hoped to attract 3,000 participants and 50,000 tourists to FESTAC, although that number was pared down in the general tightening under Fingesi. Two hundred and ten black American participants, including veteran dancer Pearl Primus, arrived by air charter - paid for by the State Department after a lengthy dispute on the amount of funds to be contributed - earlier this week.

Jeff Donaldson, FESTAC's North American zone leader and former Howard University chairman of fine arts, stayed home in the United States to try to raise funds for at least two other charter flights for performers.

There were indications that Donaldson made his fund-raising task no easier by adopting a radical black line, reminiscent of the 1960s, scarcely in favor with the more conservative black establishment that could have helped find foundation backing for him. Nor did the Black Caucus in Congress warm to his efforts at a time when black unemployment seemed a more vital issue than FESTAC.

Missing from the original list of American participants are singer Stevie Wonder, author James Baldwin, Alvin Ailey and his dance group and other established stars. At a somewhat similar cultural festival held in Dakar, Senegal, in 1966, Baldwin was top of the bill along with the late Louis Armstrong and Duke Ellington.

Also cut back for economic reasons were FESTAC plans to display hundreds of African art objects from all over the world. Insurance premiums proved to be too expensive, in part because of Nigeria's well-established underworld and piracy which is flourishing to the extent that the government recently ordered a dawn-to-dusk curfew for the ports of Lagos, Calabar and Port Harcourt.

Because of the security and insurance problems, France scrapped plans to send a number fo Picasso's Cubism works that clearly showed the Spanish master's debt to African art.

The British Museum also invoked insurance in demanding a several-million-dollar premium for the temporary return of a 16th-century Benin ivory mask which was adopted as FESTAC's emblem. The British Museum's refusal was apparently also motivated by fears that if Nigeria were given the mask, originally obtained by the British governor of the Niger coast protectorate in 1897, a precedent would be established for the return of other works of art collected during Britain's long imperial reign.

But even whittled down, FESTAC's real costs remain a mystery, despite feisty newspaper demands that the country's leaders publish a detailed account of where the money went.

FESTAC's most visible physical reminders are the colassal festive hall, a giant parade ground inspired by Ghana's impressive Black Star Square in Accra, and FESTAC village.

The village was designed to house participants and tourists and will be turned into low-cost housing when FESTAC is over Feb. 12. It is doubtful that the village, located several miles outside the city, could handle the 22,000 people it was planned for, judging by reports from its FESTAC guests who complained that some buildings are without water or gas for heating and cooking.

Despite the government's strenous efforts to build the facilities and to clean up the city, outspoken press comments doubtless mirror public disenchantment by pointing out that FESTAC officials have apparently purposely not informed Nigerians about the cost or exact dates of various events, of if indeed, the public will be admitted. With tongue in cheek, the Daily Times - government-run and the largest newspaper - chided the FESTAC organizers for "discovering some secret way of defending Nigeria's image without enlisting the help and approval of Nigerians."

Some of Nigeria's top intellectuals and artists have boycotted FESTAC, frequently for complicated reasons. But for every Wole Soyinka, a recognized dramatist said to be staying away, there is someone like the equally respected writer Cyprian Ekwensi, backing the festival.

The serious centerpiece of the festival is a three-week colloquium bringing together African specialists to discuss the overall theme of black civilization and education. There are various seminars ranging from the arts, philosophy and literature to languages, religion, sciences, government and the mass media.

Less esoteric events involve movies, music, drama and dancing (with bras on, the prudish organizers decided), and such crowd-pleasers as a 6,000-participants canoe regatta on Lagos Creek and a "durbar," or "horsemans' festival, in northern city of Kaduna.

Dedicated to the "revival, resurgence, propagation and protection of black and African cultural values and civilization," FESTAC at the very least will mark Africa's efforts to reassure itself about its roots and unity, and to assert the cultural relationship of blacks the world over.