Seventeen young people were found dead at a nightclub in the South African city of East London on Sunday, with three others dying later in hospital, officials said.
Authorities are trying to establish the cause of death as none of the bodies bore signs of injury.
"We got a report about 17 [people] that died in a local tavern in Scenery Park which is based in East London," said Brig Thembinkosi Kinana, police spokesman in Eastern Cape province.
"We are still investigating the circumstances surrounding the incident," he told AFP.
Later on Sunday, head of the provincial government safety department Weziwe Tikana-Gxothiwe said the death toll had risen to 20.
"The number has increased to 20, three have died in hospital. But there are still two who are very critical," she told local media.
Unathi Binqose, an official at the provincial community and safety department, said there was no indication that a stampede caused the deaths.
"It's difficult to believe it's a stampede as there are no visible open wounds to those dead," Mr Binqose told South African channel Newzroom Afrika
"Maybe a stampede would have happened afterwards when people saw others were collapsing ... which may have led to the death of some, but we're finding it hard to believe that some have died due to a stampede."
Regional newspaper DispatchLive reported that the bodies were found "lying strewn across tables, chairs and on the floor; with no obvious signs of injury".
Mr Binqose said some of those found at the scene were under the legal drinking age of 18. He said he understood the patrons were pupils "celebrating 'pens down', a party held after writing [high school] exams".
The owner of the club will "have a lot to answer to", he said.
Local television showed police officers trying to calm a crowd of parents and onlookers gathered outside the venue in East London, a city on the Indian Ocean coast nearly 1,000 kilometres south of Johannesburg.
"Parents whose children did not sleep at home are gathered here and they want to enter the tavern to look for their loved ones," said Mr Binqose.
"This is a crime scene and we cannot allow anyone to walk in."