South Africa closed its border with Mozambique shortly after opening it as post-election violence in the neighbouring country escalated, leading to clashes between protesters and the police.

Authorities also warned South Africans to postpone non-essential visits to Mozambique.

Amnesty International said on Wednesday that at least 20 people have died and hundreds more have been injured and arrested since the beginning of the protests in late October.

Mozambique Election Protests
Protesters fill the street in Maputo, Mozambique (Carlos Uqueio/AP)

The border was closed this week and partially opened earlier on Thursday to allow limited movement of trucks and vehicles stuck there, but the decision was quickly reversed.

Protesters in Mozambique, organised by opposition leaders and their supporters disputing the outcome of the October 9 elections that saw the ruling Frelimo party extend its 49-year rule.

Police have reportedly fired teargas to disperse protesters in the capital of Maputo.

Videos on social media sites belonging to opposition leader Venancio Mondlane show scores of young people marching toward Maputo and barricading streets with rocks and burning tyres.

Mr Mondlane, who came in second in the presidential election with 20% of the vote, has reportedly fled the country after two of his aides were killed before the results were announced.

South Africa’s Border Management Agency said it closed its side of the border after 15 employees from the Mozambican border post crossed over seeking refuge and protection.

Mozambique Election Protests
Police deploy amid opposition protests in Maputo, Mozambique (Carlos Uqueio/AP)

“They say that the protesters are on the way and have just passed the area where they process the trucks and some of the travellers,” said agency commissioner Mike Masiapato.

“They looted everything there, they took computers and they looted the trucks, the situation is just bad,” said agency commissioner Mike Masiapato.

South Africa’s international relations minister, Ronald Lamola, called for calm and an end to the violence.

“South Africa expresses concern at the outbreak of incidents of post-electoral violence and regrets the loss of lives and destruction of property,” Mr Lamola said.

Internet services in Mozambique have been disrupted since last Friday.

The opposition has accused the ruling Frelimo of rigging the election in favour of its presidential candidate Daniel Chapo, who was declared the winner with more than 70% of the votes by the national Mozambique Electoral Commission.

While the commission declares the results, it is up to the Constitution Council to verify them and deal with any outstanding disputes, including those lodged by opposition parties Renamo and Podemos.

Mozambique authorities threatened this week to unleash the army to quell the protests, accusing the demonstrators of trying to overthrow a democratically elected government.

Human rights groups have describes the government’s handling of the protests as the worst crackdown in years.