Central Italy's latest earthquake dealt a cruel double blow, not only killing helpless people, but also destroying the very finest works of art
All It took was a rumble lasting just a few seconds, a random quirk of nature, an act of God - who knows - and the Basilica of St Francis in Assisi, one of the masterpieces of Western civilisation, lay in ruins.

Frescoes painted 700 years ago by the 13th-century painter known simply as Cimabue have almost certainly gone for ever. The great fresco cycle on the life of St Francis, by Cimabue's pupil, Giotto - which helped to define the start of the Italian renaissance - appears to have survived, although it is badly cracked. Some of the frescoes attributed to Giotto's studio, however, are either severely damaged or reduced to dust.

"This could be the greatest loss ever for art history," was the immediate reaction of the Italian art historian, Mina Gregori. That first impression was reinforced yesterday when the basilica was visited by Antonio Paolucci, the distinguished former superintendent of works of art in Florence. "The first thing that must be done is to cover the rubble piled outside the basilica. It contains fragments of Cimabue and if it starts to rain it will all turn to mud," he said. Paolucci, who is to oversee restoration work, believes it will be impossible to return the basilica to its former state.

It is still not quite clear just how extensive the damage is. With rescue workers labouring around the clock to reach the dead and wounded in the surrounding area and clear away the rubble, it will be several days before the full extent of the nightmare is known. Before any of the surviving frescoes can be touched, the walls must be reinforced, but the vault over the altar decorated by Cimabue was almost completely destroyed, and part of a vault with the "School of Giotto" frescoes known as The Four Doctors of the Latin Church fell to the floor of the basilica, leaving a gaping hole in the ceiling.

There were more tremors yesterday morning, though no more deaths were reported. At least 11 people died on Friday, four of them in the Assisi Basilica itself. Entire villages in the hills dividing Umbria from the Marches region were reduced to rubble. Thousands of people spent Friday night and last night with friends, in their cars, in tents set up by the Italian emergency services - anywhere to escape a repeat of the sense of stark horror they experienced when the tremors shook them brutally awake.

Earthquakes show no respect for human life, and no consideration for artistic and architectural glories. And the extraordinary concentration of artworks in central Italy has made it one of the most compelling holiday destinations in the world. At least two other churches in Assisi - San Rufino and Santa Chiara - were damaged, along with the Apostolic Palace and the Papal Palace. In the unfashionable but charming town of Foligno, in the valley below, important buildings including the church of San Feliciano, the town hall and the bell-tower of the cathedral suffered severe damage.

In Fabriano, in the Marches, a cornice fell off the church of San Biagio, killing a 65-year-old woman as it crashed to the ground. And there were lesser incidents affecting the cathedral in Bevagna, the dazzling polychromatic facade of the cathedral in Orvieto, a bell-tower in Rieti, and the cathedral in Urbino.

The first shock occurred at 2.33am on Friday, a quake measuring 5.5 on the Richter scale which had its epicentre in the Apennines, but could be felt as far away as Venice in the north and Rome to the south. In the remote village of Collecurti, a couple in their eighties were trapped in their bedroom and later found dead in their brass bed locked in a last embrace.

By morning the authorities were confident that the worst was over. Rescue teams spread out around the region to count the dead and help the injured and homeless, while in Assisi a team of technical experts, art historians and local politicians gathered in the Upper Church of the Basilica to assess the damage to two of the Giotto frescoes and one by Cimabue.

Then, at 11.42am, without warning, the second quake happened. This one was even bigger than the first, measuring around 5.8 on the Richter scale, and its effects were all the more dramatic. Many of the images were captured on television, including a graphic sequence showing a family in Colfiorito complaining that they had been denied access to their damaged house by the emergency services, and then turning around to see their entire house crumbling to the ground.

In the Assisi Basilica, two ceiling vaults caved in, killing two Franciscan friars, and two technical experts from the local office of the Culture Ministry. The other 20 or so people who were in the Upper Church as the second earthquake struck emerged into daylight covered in thick grey dust. "My eyes were dark, and in my ears I could hear the sounds of a world collapsing around me. I don't know how I got out of there," said Plinio Lepri, a photographer with the Associated Press.

For The rest of the day, firemen and disaster relief workers laboured with mechanical diggers to move the rubble to the lawn outside the Basilica and look for signs of any further victims. Unfortunately for the restoration of these works of art, the need to move the debris so soon will have inflicted further damage on the frescoes, and made it harder to piece the salvageable ones back together again. Paolucci reported that offers of help have already been received from the British Museum and the National Gallery, and the Italian government has promised L50bn (pounds 16.2m) towards restoring the basilica. Paolucci hopes that will be complete by 2000.

The disaster has already started up old arguments, in particular the one about the wisdom of replacing a number of wooden roof struts with concrete back in the 1960s - a move which, according to the art historian Federico Zeri, made an already weak structure much more vulnerable to earthquake damage.

Some are ready to revive an even older debate, over the wisdom of building the Upper Church in the first place. In the 13th century, Franciscans were aghast as they watched the construction of a triumphalist church in the name of their master - a betrayal, in their view, of his life of charity and personal humility. "The place is cursed," said the film director Franco Zeffirelli, who shot his Brother Sun, Sister Moon in Assisi in 1970. "A priest once told me that Francis would never have wanted this church, and that sooner or later it would fall down."