Nigeria's presidential race sparked to life on Thursday with the announcement by Senate president Bukola Saraki that he intends to run next year.Saraki is the most well-known challenger yet to President Muhammadu Buhari, the former military dictator who won office in 2015 with vows to fight widespread corruption and Boko Haram extremists in Africa's most populous nation. Both have proven to be difficult tasks.The 75-year-old Buhari has repeatedly faced questions about his health after extended stays in Britain for health treatment, while offering few details. He has said he will run for a second term."Why are we not growing? There is no time to waste," the 55-year-old Saraki said while announcing his candidacy during a speech to young politicians in the capital, Abuja, to cheers. "I have decided to answer the call."Saraki, Nigeria's third most senior politician, late last month defected from the ruling All Progressives Congress party to the opposition People's Democratic Party, which he left four years ago. He met with former president and Buhari rival Goodluck Jonathan on Wednesday.Days after Saraki's defection, Nigeria's political turmoil led to a dramatic standoff in the National Assembly early this month as agents of the Department of State Services prevented lawmakers from entering. Saraki called it "an act of cowardice by those seeking to carry out an illegal impeachment of the leadership of the Senate."Vice President Yemi Osinbajo, acting as president while Buhari was again in Britain, quickly fired the head of Nigeria's intelligence agency.* Sign up to News24's top Africa news in your inbox: SUBSCRIBE TO THE HELLO AFRICA NEWSLETTERFOLLOW News24 Africa on Twitter and Facebook.