The Swaziland Solidarity Network says 38 virgin girls were killed in an accident while they were on their way to cut reeds for the annual Umhlanga reed dance.
Another 20 were seriously injured. The accident occurred along the Mbabane-Manzini highway, it said.
In a statement on Saturday, the group said: "What saddens our network is the fact that the loss of life was avoidable. To begin with, carrying people in trucks is against traffic laws."
Reports say the girls were at the back of an open truck when the collision occurred.
The group further went on to call for the reed dance to be cancelled, and for the royal family to help the respective families bury their children.
The Times of Swaziland reported that the accident involved two trucks carrying the girls, and a Toyota van. Witnesses reportedly said traffic police officers pulled over the Toyota and the first truck smashed into it. The second truck crashed into the back of the first truck.
EWN reported that the accident happened just a day before King Mswati the third was due to choose a new wife.