South African mourners expressed anger and despair today at the death of 21 teenagers in a tavern over the weekend as the pub's licence was revoked.

Investigating authorities said the youths were probably accidentally poisoned by something they ate, drank or smoked.

Seventeen died inside the bar, while four died in hospital.

Thirty-one others were hospitalised with symptoms including backache, tight chests, vomiting and headache, officials said.

The still unexplained deaths of the teens, some of whom were celebrating the end of school exams and others a birthday party, have brought an outpouring of grief and shocked a nation accustomed to injuries linked to a binge drinking culture.

Authorities ruled out an earlier-touted notion of a stampede.

Residents of Scenery Park, on the edge of East London, said they had asked authorities to close Enyobeni Tavern down weeks ago because it was serving under-aged kids.

Emergency services at the scene

At a prayer meeting at a local church, mourners and priests sang, prayed, drummed on hymn books and cried over the loss of the youngsters.

"I'm so devastated. We are so angry," Maxhabiso Sibotoboto, 50, whose 17-year-old granddaughter, Monelo, was among the dead, told Reuters by phone from a church memorial service for them.

"People were complaining about the tavern. No one was happy about it. The community wanted the tavern to be shut," he said.

Ms Monelo did not live with her parents, because he father had died and her mother worked a long way from home, he said.

Pictures circulating on social media, still unverified by authorities, showed the bodies of youths lying scattered across the floor of the tavern, some also seen motionless on tables and couches.

"We are so heartbroken, guys. We lost one of our family members, a child who was doing grade 12 (final year) this year. We can't accept it," said Yandiswa Ngqoza, an aunt, as she choked back tears outside the morgue dressed in black attire.

Morgue workers at the scene

"By the look of the child she has no visible injuries," Ms Ngqoza said, before breaking down sobbing.

Minutes earlier she had entered the morgue and identified her niece, whose name she did not disclose.

Eastern Cape police spokesperson Brigadier Tembinkosi Kinana told Reuters that the youngest victim was a 13-year-old girl.

An emotional Police Minister Bheki Cele had to pause speaking as he started weeping yesterday, triggering cries among his audience outside a morgue.

"It is either something they ingested which will point to poisoning, whether it is food or drinks, or it is something they inhaled," Unathi Binqose, spokesperson for Eastern Cape's provincial community safety department said by phone today, ahead of a toxicology report still being awaited.

He said hookah pipes were visible in CCTV footage of the scene.

"We are ruling out a stampede," he added, after initial media reports suggested this may have been the cause.

Sinovuyo Monyane, 19, who was hired by the bar to promote an alcohol brand, said she was still "confused" but felt lucky to be alive.

She said she struggled to escape through a door gridlocked with people.