JOHANNESBURG (AP) — South Africa closed its border with Mozambique shortly after opening it on Thursday. Post-election violence increased in neighboring countryClashes broke out between protesters and police.

Authorities also warned South Africans to postpone non-essential visits to Mozambique. At least 20 people have been killed and hundreds more injured and arrested since the protests began in late October, Amnesty International said on Wednesday.

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

Protesters discussing the outcome of the 9 October election, which was held and decided by opposition leaders and their supporters in Mozambique Frelimo party extended its 49-year rule. In the capital of Maputo, police reportedly used tear gas to disperse protesters.

Videos on social media sites owned by opposition leader Venancio Mondlane show scores of young people marching towards Maputo and barricading the streets with stones and burning tires.

It was reported that Mondlane, who came second in the presidential elections with 20 percent of the votes, later fled the country. two of his assistants were killed Before the results are announced.

The South African Border Management Agency said it closed its side of the border after 15 workers from the Mozambique border post crossed the border seeking asylum and protection.

“They say the protesters were on the road and had just passed the area where the trucks and some passengers were being processed. They looted everything in there, took computers and looted trucks, it’s very bad,” said Mike Masiapato, the agency’s commissioner.

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

“South Africa expresses concern about the outbreak of post-election violence and regrets the loss of life and destruction of property,” Lamola said.

Internet services in Mozambique have been disrupted since last Friday.

The opposition accused the ruling Frelimo of rigging the election in favor of presidential candidate Daniel Chapo, who was declared the winner by the national Mozambique Electoral Commission with more than 70 percent of the vote.

While the Commission announces the results, it is the Constitutional Council’s responsibility to verify them and deal with unresolved disputes, including disputes put forward by the opposition parties Renamo and Podemos.

Mozambican authorities this week accused demonstrators of trying to overthrow a democratically elected government and threatened to unleash the army to suppress the protests. Human rights groups have described the government’s handling of the protests as the worst crackdown in years.