By Sudipto Ganguly

AHMEDABAD, India (Reuters) -Dozens of anxious family members sat outside an Indian hospital waiting to collect bodies of loved ones killed in Thursday's Air India plane crash, as doctors worked overnight to gather dental samples from the deceased and run identification checks.

In the world's worst aviation disaster in a decade, an Air India Boeing 787-8 Dreamliner with 242 people on board bound for London took off from Ahmedabad, but descended and crashed within about 30 seconds, erupting into a massive fireball.

Outside the B.J. Medical College in Ahmedabad, an elderly woman said four of her relatives including two children were onboard the flight, but declined to speak further to the media until the bodies were handed over. 

"Can you give us the dead bodies? If not then we will not give interviews. We are so tired now," she said in frustration.

Other relatives sat patiently at the hospital where many have in recent hours given blood samples for DNA profiling at a dedicated centre for collection.

At the hospital, Jaishankar Pillai, a forensic dentist, told reporters the doctors were in the autopsy room until 4:30 a.m. on Friday collecting dental samples, as "teeth can withstand the heat", and they hoped they could use them for identification.

"We have recorded the dental records of 135 charred victims ... it's a very pathetic situation," said Pillai, adding he did not have data for how many bodies had been identified so far.

Officials outside the autopsy room told Reuters at least seven bodies had been handed over to their relatives after identification checks.

In the case of dental records, a person is not typically identified based on a relative's teeth, but through reference to the victim's prior dental charts, radiographs, mouth guards or other records.

Pillai added that even a selfie photograph of the victim could help doctors match the gap between two teeth to run checks. 

Scenes of distress played out beside the autopsy room. Daksha Patni was mourning the loss of her relative, 14-year-old Akash Patni, and wailing as she waited for his body.

The cause of the crash, the first for a Boeing Dreamliner wide-body airliner, has not yet been determined and India's aviation minister said a formal investigation had begun. 

A family member of another victim, 81-year-old Abdur Razzaq Chitthi Wala, told IANS news agency he was not being allowed to verify the body. 

"I received a video showing his body, it's burnt, but the face is clearly visible. All I'm asking is to let me verify the body," said the relative, who did not share his name during the interview.

"They are saying give your blood sample, and you will get a call."

(Writing by Aditya Kalra, Editing by William Maclean)