* Bus slams into cars before crashing through road barriers
* Prosecutors investigate; driver killed
* Crash comes four days after Spanish rail disaster
Thirty-eight people were killed and at least 18 injured when a bus plunged 25 metres (80 ft) off a viaduct in Italy, in the second major transport disaster in southern Europe in less than a week.
Initial reports suggested that the coach was travelling at high speeds and hit four or five cars before crashing over the roadside barriers on a stretch of road near Monteforte Irpino, east of Naples, on Sunday night. There were around 50 people on board, many of them children.
It was one of Italy's worst road accidents and came just five days after 79 people were killed when a high-speed train derailed in the Spanish pilgrimage town of Santiago de Compostela.
Prime Minister Enrico Letta, on an official trip to Athens, cancelled a private visit to the Acropolis.
"It is a very sad day for Italy, what happened last night. There are no words for it," he told reporters. "It is a huge tragedy."
Italy's government has declared Tuesday a day of national mourning, when the funeral of the victims will take place.
Pope Francis said in a telegram he shared the pain the Campania region was suffering and he prayed for the victims and for the injured to recover quickly.
A statement from motorway operator Autostrade per l'Italia said the coach appeared to have been travelling fast near slower-moving traffic, even though a lower speed limit had been clearly indicated.
Officials were examining onboard instruments and the guardrail on the highway where the accident occurred to try to determine the cause of the crash.
"You would think that the barriers on the viaducts and bridges should prevent this type of accident but evidently it seems the impact was so strong that even the barrier gave way," said Alessio Barbarulo, head of the local fire brigade division that coordinated the rescue effort.
As rescue services worked, the crushed wreckage of the coach lay on its side. Bodies covered in white sheets were lined up on the road before being taken away by ambulances.
There was some initial confusion over the number of casualties but a police official said 38 people died in the accident. Thirty-five were killed immediately and another three died later in hospital, he said.
Eighteen people, including a number who were travelling in cars struck by the coach, were injured.
Infrastructure Minister Maurizio Lupi said it was too early to say what may have caused the accident but that there was no indication of technical problems with the bus, which had passed its annual inspection in March.
Witnesses quoted by Italian media said the bodywork on the coach appeared to have been damaged as the bus sped along the highway before the impact, suggesting that the driver may have experienced problems controlling the vehicle before the crash.
An official from the prosecutor's office in nearby Avellino said investigations would look at the possibility of manslaughter or causing an accident.
Magistrates are investigating the role of motorway operator Autostrade per l'Italia among others, the official said. Autostrade per l'Italia declined to comment.
The driver was killed in the crash and an autopsy has been ordered on his body, officials said.
All of the victims appear to have been Italians returning from an excursion to the town of Telese Terme, known for its hot sulphurous springs and a nearby lake popular as a fishing spot.
Local media said the stretch of road where the bus crashed had been the scene of repeated accidents but Autostrade per l'Italia said the rate of accidents in the area was lower than the national average.