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.

Civil society groups and Western observers also said the election was unfair and that results were altered. A Frelimo spokesperson did not respond to requests for comment.

The party has been accused of vote-rigging in previous polls and has denied it, sparking smaller protests last year.

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.

A Reuters witness saw a police officer firing a handgun in the direction of a crowd of protesters in a Maputo township.

"There has never been something like this in the country," said Adriano Nuvunga, director of Mozambique's Centre for Democracy and Human Rights.

"Whatever the outcome... the young people have gained their voice," he said.

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.

On Tuesday it ordered the electoral commission to clarify within 72 hours why there had been discrepancies in the number of votes counted in the presidential, legislative and provincial elections, according to a letter seen by Reuters.

An electoral commission spokesperson did not immediately respond to a request for comment.

"(The outcome) comes down to what the Constitutional Council decides, I suspect. But the anger is palpable and I don't know what they can say that will calm things down," said Louw Nel, a political analyst at Oxford Economics Africa.

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.