Police have fired tear gas at protesters in Mozambique in the biggest demonstration yet against the long-ruling Frelimo party, which was declared the winner of disputed elections last month.

Anger has mounted since election authorities said Frelimo won the October 9 vote, extending its 49 years in power. Human rights groups said at least 18 people have been killed in the police crackdown on protests since then.

The general election was hotly contested with many young people supporting independent presidential candidate Venancio Mondlane, who said 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 results were altered.

On Thursday, police used tear gas to disperse thousands of protesters in the capital, Maputo.

Mondlane – a 50-year-old former radio presenter who, according to official results, lost to Frelimo’s Daniel Chapo – said it was a “crucial moment” for the country.

“I feel that there is a revolutionary atmosphere … that shows that we are on the verge of a unique historical and political transition in the country,” Mondlane said from an undisclosed location. He said he could not disclose his whereabouts other than to say he was not in Africa.

The Mozambique Bar Association warned there were “conditions for a bloodbath” on Thursday as a heavy security presence was deployed across the capital. It is home to more than one million people, but it resembled a ghost town as shops, banks, schools and universities were closed.

Authorities have restricted internet access across the country in an apparent effort to “suppress peaceful protests and public criticism of the government”, according to Human Rights Watch.

United Nations High Commissioner for Human Rights Volker Turk also warned against “unnecessary or disproportionate force”, saying police should “ensure that they manage protests in line with Mozambique’s international human rights obligations”.