Mozambican police fired tear gas at protesters in the capital Maputo on Thursday in the biggest demonstration yet against the long-ruling Frelimo party, which was declared the winner of a disputed election last month.
Anger has mounted since election authorities said Frelimo had won the October 9 vote, extending its 49-year rule.
Human rights groups say at least 18 people have been killed in the police crackdown on protests since then.
The election was hotly contested, with many young people supporting independent candidate Venancio Mondlane, who says the vote was rigged and called for a week of protests culminating on Thursday.
Biggest demonstration yet
Police were deployed in full force on Thursday as large groups of mostly young men barricaded streets with burning tires and waved homemade signs in support of Mondlane.
"There has never been something like this in the country," said Adriano Nuvunga, director of Mozambique's Centre for Democracy and Human Rights.
President Filipe Nyusi has not spoken since the protests escalated. His defence minister has threatened to deploy the army and warned against attempts to grab power.
Mozambique's Constitutional Council has not yet certified the election results, a process that usually takes around two months.
Border closure
Meanwhile, South Africa has closed its main border crossing for security reasons, and its foreign ministry advised its citizens against all but essential travel to Mozambique.
South African logistics company Grindrod on Thursday said it had suspended port and terminal operations in Mozambique.
South Africa closed its border with Mozambique shortly after opening it on Thursday 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.
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 tires.
Mozambique authorities threatened this week to unleash the army to quell the protests, accusing the demonstrators of trying to overthrow a democratically elected government.
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