Newly inaugurated Mozambique President Daniel Chapo during the final Frelimo election campaign in Matola, Mozambique, on October 6. Photo: Reuters/Siphiwe Sibeko

A civil society monitoring group said more than 300 people have been killed in clashes with security forces since the October 9 vote, which the opposition said Frelimo won through vote-rigging and Western observers said was not free and fair.

Frelimo denies accusations of electoral fraud. It has governed Mozambique since the end of the war against Portuguese colonial rule in 1975, clinging on throughout a 15-year civil war that killed a million people before a 1992 truce.

Chapo told a group of about 1,500 supporters from a stage in the capital Maputo that social and political stability would be his government's priority. He also promised to shrink the size of the government by reducing the number of ministries, tackle youth unemployment and prioritise health and education.

The city centre was largely deserted with a heavy police and army presence, witnesses said.

South African President Cyril Ramaphosa was one of the few heads of state attending Chapo's inauguration.

Opposition leader Venancio Mondlane, who official results say came second to Chapo in the presidential election, returned from self-imposed exile last week and has urged his supporters to continue demonstrating.

The post-election protests amount to the largest against Frelimo in Mozambique's history and have affected foreign businesses operating in the resource-rich southern African country of 35-million people. They have also disrupted cross-border trade and forced some to flee to neighbouring countries.