Nigerian President Muhammadu Buhari was sworn in for a second term in office on Wednesday, vowing once more to tackle crippling security threats and root out corruption in Africa’s key economy.

The 76-year old leader, in power since 2015 and re-elected in February, took the oath of office for a second four-year term in the capital Abuja.

« I do solemnly swear that I will be faithful and bear true allegiance to the Federal Republic of Nigeria, » Buhari said, dressed in simple white robes and traditional embroidered cap. « I will preserve, protect and defend the constitution. »

Buhari took the oath of office at what officials called a « low-key » ceremony.

It included red-carpet arrival flanked by bagpipers into a stadium packed with dignitaries and military guard of honour.

Vice President Yemi Osinbajo was also sworn into office.

Buhari was re-elected with 56 percent of the vote in Africa’s most-populous nation — and top oil producer — after a delayed poll that angered voters.

His rival, Atiku Abubakar of the Peoples Democratic Party (PDP), who came second with 41 percent of the vote, has along with other opposition leaders launched an ongoing legal challenge to the victory.

They allege irregularities in the vote and have called it a « sham » result.

Buhari, a former army general who led a tough military government in the 1980s, campaigned on a promise to make the country safer.

He begins a final four-year term beset with numerous challenges.

Nigeria is struggling from multiple conflicts, including an Islamist insurgency in the northeast of the country.

His time in power has also been dogged by questions about his medical fitness. He has spent several months abroad for treatment for an unspecified condition.

Buhari has touted himself as a « converted democrat » to persuade those with misgivings that his military past was history.

But in office he has struggled to shake off claims of authoritarianism — particularly in his fight against corruption which critics say has been one-sided against perceived political opponents.