After a civil war that killed around 1 million people before a 1992 truce, and periodic violence since, Nyusi signed a deal with rebels-turned-opposition party Renamo in August meant to bring definitive peace to a country on the cusp of becoming a global gas exporter.

Instead, October's election exacerbated decades-old wounds: Nyusi, of the ruling Frelimo party, won with over 70% of votes but his main rival Ossufo Momade said the poll was rigged.

Neither Momade nor any Renamo lawmakers turned up for the swearing-in at Independence Square in central Maputo.

Speaking after the ceremony, Nyusi said over his next five-year term he would seek to ensure peace lasts. "Peace has been and will be our absolute priority," he told the crowd.

Renamo secretary general Andre Magibire told Reuters the party does not recognize Nyusi as legitimate leader, but would also not be drawn back into violence.

"There's no way we can derail the peace deal, even though Frelimo keeps pushing us toward war," he said on Tuesday.

Frelimo spokesman Caifadine Manasse dismissed that.

"The opposition is crying foul, in general it uses this tactic to spark instability." the spokesman said.

Renamo is fracturing. A breakaway group of former fighters have been staging attacks in former heartlands.

This low-level fighting threatens to draw resources away from efforts to tackle a festering Islamist insurgency in the north, on the doorstep of blockbuster gas projects led by Exxon and Total.

Mozambique's top court dismissed Renamo's challenge against the results in November.