Deep in a trance and impervious to the heat of burning coals underfoot, lithe young men prance across piles of glowing embers at a ritual fire dance to exorcise spiritual demons and pray for a healthy harvest.

Once dismissed as superstitious and banned by Vietnam's authorities, the fire dance is now performed publicly by the Pa Then ethnic minority after decades when they celebrated in the woods in secret.

"Everyone here is so excited to see it," said first-time observer Hua Manh Linh, who joined hundreds to watch the hours-long nighttime ritual by the animist Pa Then people in northern Tuyen Quang province.

The evening starts with an offering to the gods: a whole boiled hog atop a platter of its own intestines. Logs are stacked in a two-metre (six-foot) tall tower ready to be lit nearby.

The offering of a boiled pig sitting on a platter of its own intestines starts the Vietnamese minority Pa Then's ritual fire dance. Photo: AFP

The shaman leading the ceremony taps a traditional string instrument to invoke spirits.

Soon the dancers are possessed: their eyes glaze over, their bodies jerk around, and they say the spirits shield them from the hot coals they are about to leap onto.

"It feels like jumping into a bath, and when the priest asks us to stop, we stop, otherwise we'll get burned," said Ho Van Truong, who has danced in the past, though not this year because he said couldn't invoke any spirits.

For him, the ceremony is a proud display of Pa Then culture, defined by a belief that everything in the universe possesses a soul.

The dance was driven underground in the 1960s and 1970s as local authorities cracked down on so-called superstitious rituals, according to historian Nguyen Van Huy.

But since the country started opening up in the late 1980s, the ritual crept back into public life.

Today it is listed as intangible cultural heritage by the government, and performed at museums and ethnic minority festivals.

But Huy warns that turning the dance into a public spectacle risks cheapening its sanctity.

"(It's) now getting more popular, and is performed more and more, so I worry about its sacred value, whether it will be preserved or lost in time," said Huy, author of "Cultural Mosaic of Ethnic Groups in Vietnam".

Pa Then shaman Sin Van Phong says his concern is that younger generations, perhaps more rapt with mobile phone apps than centuries-old rituals, will let the fire dance fade.

"The biggest challenge is passing the job on to younger generations, young people don't want to learn about the ceremony now," he said.