South Africa's border authority said on Saturday it had partially reopened its main crossing with neighbour Mozambique, where last month's disputed election sparked protests, raising safety concerns.

Opposition supporters in Mozambique have been demonstrating against what they say is a fraudulent win by Frelimo, the party that has ruled the southern African country since 1975.

South Africa closed the Lebombo port of entry in its Mpumalanga province earlier this week after receiving reports of vehicles being torched on the Mozambican side.

"The port is now open for cargo movement, after the Mozambican authorities managed to address challenges with their systems to facilitate processing through the Ressano Garcia port of entry into Mozambique," the Border Management Authority said.

It said in a statement that the initial reopening of the port was to manage the movement of people, adding: "but this morning, the Mozambican authorities worked tirelessly to address cargo systems for smooth border processing".

At least 21 people have been killed in Mozambique and scores injured in protests in which demonstrators have burned tyres, blocked roads and marched to Frelimo offices.

Authorities have responded with tear gas and live ammunition, human rights groups said, and restricted access to the internet.