Decades ago, Mozambique’s liberation party, Frelimo, easily attracted adoring crowds. The promise of salvation from Portuguese colonizers, and a life with jobs and housing for all, was an easy sell in a southern African nation that was suffering under racist rule.

But when Daniel Chapo of Frelimo becomes president on Wednesday, he will confront a country more dissatisfied with his party than at any point during the country’s 50 years of independence. Millions of people took to the street after the October election, which voters, international observers, opposition leaders and rights groups have roundly criticized as fraudulent.

The country of 33 million has been roiled by political chaos since the vote. And now, Frelimo’s grip on power is being tested like never before at a time when Mozambique faces urgent economic and social crises, analysts say. Two of the three opposition parties boycotted the opening of Parliament on Monday.

The anger among voters exploded into massive street protests in the past several months that led to clashes with the police. At least 300 people have been killed.

Mr. Chapo and his party had likely hoped that the inauguration on Wednesday would help move the country toward reconciliation and stability. Instead, the challenges facing Frelimo’s leadership could just be getting started, political insiders say.