South Africa has temporarily closed its main border crossing with Mozambique over safety concerns, its border authority said on Wednesday, as protests against last month's disputed election in Mozambique continued to escalate.

At least 18 people have been killed in the protests, according to human rights groups, as opposition supporters demonstrate against what they say is a fraudulent election win by Frelimo, the party that has ruled Mozambique since 1975.

Security forces have responded with tear gas and live ammunition while the government has restricted internet access and threatened to deploy the army.

South Africa has closed the Lebombo port of entry in its Mpumalanga province after receiving reports of vehicles being torched on the Mozambican side, the authority said in an emailed statement to Reuters.

"Due to these security incidents and in the interest of public safety, the port has been temporarily closed until further notice," it said, adding that seven Mozambican officials had requested refuge in South Africa for safety and protection.

A reporter for South Africa's public broadcaster SABC posted a video on X showing a long line of trucks which he said stretched for kilometres waiting to cross the border.

The latest protests in Mozambique are part of a week of demonstrations called by presidential candidate Venancio Mondlane, who came second in official results for the October 9 election but claims to have won.

The electoral commission has declined to comment on allegations of fraud and Frelimo has not replied to requests for comment.

The protests are set to culminate in a march on the capital Maputo on Thursday. Mozambique's Defence Minister Cristovao Chume said on Tuesday the military was ready to intervene and warned against attempts to grab power.

The Mozambican Centre for Democracy and Human Rights said that 24 people had been killed due to police violence. Human Rights Watch researcher Zenaida Machado told Reuters that she had verified 18 deaths so far but believed there had been more.

The interior minister has defended the police response, saying the protests were violent and that security forces were compelled to restore public order, which resulted in deaths.