Albino people live in fear in Tanzania and Burundi
Seven people have gone on trial in Tanzania accused of murdering albino people and selling their body parts for use in witchcraft.
More than 40 albinos have been killed there in the last 18 months.
Witchdoctors sell their albino potions for thousands of dollars, claiming they bring prosperity and good fortune.
Powerful businessmen are believed to be among the clients driving the trade. Dozens of people have been arrested, but none convicted so far.
Human rights organisations have expressed shock it has taken so long for the trials, in Shinyanga and Kahama in the north of the country, to begin.
BBC East Africa correspondent Will Ross says the justice system in Tanzania is notoriously slow and corrupt.
In Shinyanga, in the north-west, four men pleaded not guilty to carrying out a gruesome murder.
They were accused of attacking a young albino woman, cutting off her head and limbs which were stuffed into a plastic bag and then dumping her torso in a well.
In nearby Kahama, three men also pleaded not guilty to killing a 13-year-old albino boy and severing his legs.
Our correspondent says prejudice against people with albinism is widespread in Africa. But ritual killings in such numbers is a new phenomenon.
Both adults and children have been attacked in their homes, hacked to death or had their throats cut. Each body part can be worth thousands of dollars.
Superstitious miners, fishermen and businessmen in Tanzania, hoping to get rich quick, have been accused of fuelling the demand for the potions.
The number of attacks has gone down in recent months, but the albino people still live in fear.
When a person with albinism dies and is buried, their resting place often has to be cemented over to deter grave robbers.
The Tanzanian government has publicly stated its desire to end the killings. In March, President Jakaya Kikwete called on Tanzanians to come forward with any information they might have.
Last month, a similar trial began in neighbouring Burundi, with 11 men accused of attempting to killing albino people and selling their body parts. Some were believed to have been traded over the border to Tanzania.
There are estimated to be about 17,000 albinos living in Tanzania. They lack pigment in their skin and appear pale.