Architecture: Walls came tumbling down: Jonathan Glancey, our man in Havana, finds one of the most beautiful cities in the world crumbling under a US trade embargo

To say that Havana is collapsing is to state the obvious. It is even more obvious when the building you are about to enter falls down in front of you. One moment Victor Marin, one of the city's leading architectural conservationists, is reciting the faded glories of the arcaded 18th- century merchant's house that occupies the north-west corner of Plaza Vieja, one of the oldest and most elegant city squares in the Americas. The next, the building crumbles and falls as quickly and quietly as a house of cards.

A cloud of acrid dust smothers us. We run into the still shifting mound of stucco, stone, wrought iron and rubble, accompanied at first by just the sound of our shoes and then by a chorus of screams and cries, to give what useless help we can. As if Wagner were directing the scene, the skies crack open, the dust settles and the citizens of Havana are drenched in an operatic tropical storm. Terrific thunder and lightning are accompanied by a leitmotif of klaxons and flashing lights - rescue services arrive within five minutes.

No one died in the collapse on Plaza Vieja last Tuesday. Neither, remarkably, was anyone killed later that evening when a second building imploded on the Malecon, the dual carriageway that sweeps along Havana's sea-front and knits together the various parts of the Cuban capital. 'In this weather (86 degrees, 100 per cent humidity and frequent storms),' Marin says, 'we can expect there to be several more collapses.'

In the sanctuary of the 17th-century Convento de Santa Clara - the exquisite national centre for the restoration of Cuba's artistic heritage - one of Marin's colleagues congratulates me on my escape from death by architecture. He then tells me that funding for the restoration of Plaza Vieja was to have come from Unesco. Money had been set aside.

But, after a meeting three years ago between Margaret Thatcher and Ronald Reagan, fresh trade sanctions were imposed on Castro's Cuba and funds for saving Plaza Vieja were withdrawn. Indirectly, the economic policies of Britain and the United States (what free trade?) have destroyed one of the finest buildings in the Caribbean as well as nearly killing Victor Marin and some of the most talented conservationists to be found anywhere in the world. And me.

Cutting funds for the restoration of old Havana was a particularly cruel act. If the Cubans can see one way out of their desperate economic plight - and it is getting more desperate by the day - it is, for better or worse, through tourism.

Old Havana is, in parts, one of the world's most exquisite cities (Juliet Barclay's beautiful book Havana, Portrait of a City, with photographs by Martin Charles, published by Cassell at pounds 20 this month, celebrates the magnificent restoration work that has been carried out to date). Yet most of the old city remains in a state of decay, and visitors need more than rose- tinted glasses to overcome the feeling that they are walking around a city stuck in a Fifties time-warp that has been subjected to a prolonged and vicious bombardment. The rest of the city - save for the tree- lined suburbs of Miramar and Cubanacan some way west - is not for the eyes of tourists.

Cuba hopes to earn dollars 1bn a year from tourism by 1996. Much of this will come from package tourists - mostly from Spain, Italy, Canada and Germany - who come here to soak up the sun at the new Miami-style, dollar-only beach resorts at Varadero, built by foreign investors, mostly Spanish and Mexican. The architecture of these is fit for their purpose: uniformly Post-Modern and garish, this is where sun-seekers can find cheap Caribbean holidays. The girls - and boys - come even cheaper (a 14-year-old in exchange for a meal, Coca-Cola or foreign cigarettes), the beaches are wonderful, the cocktails are great and served to you by the humbled children of the revolution.

If dollar-rich tourists can be contained in Varadero and similar resorts, say the conservationists, the money they spend will be pumped back into Cuba's still excellent, if tottering, health and education services and into the rebuilding of Old Havana itself. If it can be rebuilt, the city should gradually attract sophisticated tourists who want a mix of sun, rum, architecture and culture. Without dollars generated by the tourist industry, Havana's future - and that of an independent Cuba - hangs in the balance.

It is difficult for tourists visiting Havana to gauge the condition of the Cuban capital. The city's main hotels are in relatively good condition and its museums, more often than not, superb. Each hotel is a hermetic, air-conditioned world from which Cubans are excluded. There is a system of virtual apartheid. Twice last week I was refused entry into hotels. Once it was for trying to bring Cubans into the Habana Libre for a cold beer. Castro made this blue and white tower - one of the tallest in Havana - his headquarters in 1959; formerly the Hilton Hotel, it was designed by the American architect Welton Beckett and opened in 1958. The Hilton has tried unsuccessfully to buy it back; it is now owned by a Spanish company. The Cubans' clothes gave them away and we were turned away.

And then I was prevented from going into the restored Plaza Hotel (Neo-Colonial, designed by Jose Mata in 1908) for looking more like a Cuban than a gringo. Outside these havens of cold beer and coffee (difficult to get elsewhere, and certainly not for pesos) and off the tourist track, Havana is a city of beautiful but fast-decaying streets and houses. The architectural legacy from the 18th century to the 1950s is exceptional. Perhaps only Ricardo Porro's romantic Escuela Superior de Arte, dating from 1965, speaks of high quality revolutionary-era architecture, and even this is falling to pieces.

In Havana - if you have the courage to trawl streets that sometimes make Mexico City look like Paris - you will discover magnificent examples of houses and civic buildings in every conceivable style: Spanish Colonial, Neo-Classical, Art Nouveau, Art Deco, Bauhaus, California ranch-style . . .

Rarely are these the homes of the wealthy. When those with money fled Havana for Miami as Che Guevara and his bearded guerrillas occupied La Cabana (the impressive 18th-century fortress that guards the port of Havana) on 1 January 1959, their houses were rapidly occupied by the working class and peasants moving into the capital. The same people still live in them. Imagine the most impoverished of families living in the grandest stuccoed streets of London's Kensington or Holland Park and you catch something of the flavour of downtown Havana.

Poverty means that these houses are all in decay, many terminal. Those who live in their tottering splendour feed their families by keeping hens and pigs indoors. Visit homes in central Havana and you will find farmyard animals snuggling up to Santeria saints, Soviet televisions and film-star portraits of Fidel and Che.

Meanwhile, families from the country still make their way to Havana in the hope of getting their hands on the US dollars they need to make ends meet. Unable to find homes (building work in Havana has all but stopped except in the tourist sector), they settle precariously in shanty towns growing along the railway tracks in the old working-class area of Cerro and in the district known as the Isla del Polvo (Isle of Dust) on the southern fringe of the capital. Here, in the dusty perimeters of the now redundant lime quarries of Pogolotti, families build shacks from whatever materials they can find; bedrooms and bathrooms are sometimes caves scooped out of mud.

Havana's collapse is sad and unnecessary. This is a world-class city that has - because of a mix of home-grown economic dogma and heavy-handed US politics - been reduced to a Third World heap. Its architects, engineers and planners are bursting with new ideas for saving and enhancing their city, but they have no money (and often no telephones, electricity, lunch, or means of getting to work).

The tropical climate, termites and tourist industry have and will always undermine the incomparable fabric. Graham Greene evoked this decay 35 years ago in Our Man in Havana. Those who love the capital of the Caribbean can do little other than take dollars over by the bagful, campaign against the trade embargo placed on Cuba by the US, hope that the Cuban government moves faster towards political and economic reform - and wait. Meanwhile, if you plan to tour the architecture of Havana, take a warm heart and a hard hat.

(Photograph omitted)