Cameroon’s long-time leader Paul Biya was sworn in on Thursday for a new seven-year term following his victory in last month's presidential election, which his opposition rival has described as "a constitutional coup."

Speaking to Parliament, the world’s oldest president promised to stay faithful to the confidence of the Cameroonian people and vowed to work for a “united, stable and prosperous” country.

The 92-year-old incumbent, who is Africa’s second-longest serving leader, took the oath of office during a session of parliament in what residents describe as the heavily militarized and partially deserted capital, Yaounde.

Cameroon’s top court on October 27 declared Biya the winner of the election, with 53.66% of the vote, ahead of his former ally-turned-challenger Issa Tchiroma Bakary, who scored 35.19%.

There were protests in several parts of Cameroon days after the October 12 vote, followed by a three-day lockdown this week after Tchiroma claimed victory and alleged vote tampering.

The government has confirmed that at least five people were killed during the protests, although the opposition and civil society groups claim the figures are far higher.

Tchiroma insists Biya was awarded a “fraudulent” victory in the election.

“The will of the Cameroonian people was trampled that day, our sovereignty stolen in broad daylight,” Tchiroma wrote on Wednesday night.

“This is not democracy, it is electoral theft, a constitutional coup as blatant as it is shameful.”

Biya came to power in 1982 following the resignation of Cameroon’s first president and has ruled since following a 2008 constitutional amendment that abolished term limits.

His health has been a topic of speculation as he spends most of his time in Europe, leaving governance to key party officials and family members.

He has led Cameroon longer than most of its citizens have been alive — over 70% of the country’s almost 30 million population is below the age of 35. If he serves his entire term, Biya will leave office nearly 100-years old.

The results of his nearly half-century in power have been mixed; armed insurgencies in both the north and west of the country, along with a stagnant economy, have left many young people disillusioned with the leader.