Relatives who suspected a British teenager of practicing witchcraft beat and drowned him Christmas Day 2010 as he "begged to die" because of the pain they were inflicting, prosecutors alleged in a London court.

Kristy Bamu, a 15-year-old boy from Newham, east London, allegedly died at the hands of Eric Bikubi and his partner, Magalie Bamu, Kristy's sister. Both of them have denied the charges.

Bikubi and Bamu allegedly forced Kristy's four siblings to join in the attack.

The alleged attackers hit the teen with sticks, metal bars, a hammer and chisel, prosecutors said, and he "begged to die" because of the pain from the beating.

"It was only when he [Bikubi] realized that he [Kristy] wasn't moving that he stopped what he was doing … by then it was too late," prosecutor Brian Altman said.

Kristy suffered more than 100 injuries.

Kristy and his siblings had arrived in London from Paris to see the East London couple for Christmas. But Kelly, Kristy's elder sister, said the trip turned sour when Eric Bikubi accused her and Kristy of practicing witchcraft and sorcery. Eric Bikubi also accused them of influencing his 3-year-old son.

"Despite her own siblings' denials that they were sorcerers, Magalie Bamu joined her boyfriend in repeating these fantastic claims and participating in the assaults," Altman alleged.

"They beat them, refusing to let them eat, drink or sleep for days while the punishments being meted out became increasingly violent, with them using many implements found in the flat as weapons of torture," Altman said.

"However, it was Kristy Bamu who became the focus of Bikubi's attention and, in a desperate attempt to prevent any further suffering, he and his two sisters were eventually to admit to being sorcerers. Wickedly, the defendants also recruited sibling against sibling as vehicles for their violence," Altman told the court.