LA TODOLELLA, Spain, Feb 7 (Reuters) - Spain was plunged into mourning on Monday after 18 friends at a birthday party were found dead, apparently due to a gas leak at a mountain hostel they rented.

It was the worst death toll in a gas leak in Spain in at least 15 years, Spanish media reports said.

The bodies were found at the remote Sant Cristofol hostel near the picturesque medieval village of La Todolella, 300 km (180 miles) east of Madrid.

"It hits all of Spain. It's a day of national mourning.

Eighteen young people have died," said Francisco Camps, head of the government of the Valencia region where the bodies were found.

"The tragedy is absolute," he told state radio.

The deaths of the 18 local people, aged between 22 and 40,

traumatised the closely knit community where everyone knows each other. Stunned relatives waited at a sports centre in La Todolella, a village of 140 inhabitants, for news.

About 50 people had gone to the birthday party at the hostel on Saturday night and the dead had stayed the night there while the others decided to return home.

The dead included a group of musicians who had played at the party, according to the Web site of newspaper El Mundo.

Two people survived who slept in another room of the hostel.

They were found unconscious on Sunday afternoon but were revived, a government official said.

The names of the victims were not released and the official could not say how many were men and how many women.

A regional government official, Alicia de Miguel, said the deaths were apparently due to a faulty gas heater that the party-goers had taken with them to warm up a cold night.

The hostel had central heating but it had broken in recent freezing weather.

The mayor of La Todolella, Alfredo Querol, said the probable cause was a leaking butane bottle.

"They had a party last night, for one guy's birthday, and some of them stayed at the hostel to sleep. All those who slept there, apart from two who were in a separate room, are dead," he told Cadena Ser radio.

A judge was working in the hostel identifying the bodies late into the night. Once identified, they were to be taken to the nearby coastal city of Castellon for autopsy.

The Valencia regional government offered psychological help to the families of the victims.