Updated

Parents in South Africa's economic hub of Johannesburg have been asked to keep their children at home from school "for their own safety" after a protest by minibus taxi drivers turned violent.

The drivers protesting the high price of their vehicles threatened motorists and blocked highways Thursday, causing massive gridlock. Police said some of the vehicles had been hijacked by taxi drivers.

Passengers concerned about missing flights were seen wheeling their luggage along a highway to O.R. Tambo International Airport as hundreds of minibus taxis blocked traffic.

Motorists have reported violence. South Africa's Automobile Association has warned drivers in the city to be vigilant and remain calm if threatened.

The South African National Taxi Council calls the high monthly payments on the vehicles it uses "simply unaffordable."