By Felix Onuah and Ulf Laessing
ABUJA/LAGOS, Nigeria (Reuters) - Nigerian President Muhammadu Buhari said on Friday he would need more rest and health tests after coming home from nearly two months of medical leave in Britain during which his deputy, Yemi Osinbajo, stamped his authority on economic policy.
Shortly after arriving back from London, the 74-year-old former general told officials he was feeling "much better" but wanted to rest over the weekend, raising questions about his ability to run Africa's biggest economy and most populous nation.
Osinbajo, a lawyer who is seen as more business-friendly than Buhari, played an active role in driving policy changes during the president's seven-week absence.
The Nigerian stock exchange <.NGSEINDEX> jumped to a one-month high when Buhari returned, but trimmed gains after his comments about his ill-health raised fears of policy confusion and a power vacuum.
Dressed in a dark kaftan and Muslim prayer cap, Buhari walked stiffly but unaided from his plane after it landed at an air force base in the northern city of Kaduna.
After greeting a handful of provincial and military officials, he boarded a helicopter to the capital Abuja to address Osinbajo and his top military and security commanders in a brief speech.
"I deliberately came back towards the weekend so that the Vice President will continue and I will continue to rest," Buhari said at the presidential villa. "All I need is to do further follow-ups within some weeks."
He said he was "conscious" of the needs of the economy, mired in its first recession in 25 years due to a collapse in oil revenues, but failed to clarify Osinbajo's role.
His spokesman Femi Adesina later said Buhari would formally notify parliament on Monday that he was back in charge. He had made Osinbajo acting president during his absence.
Buhari has not revealed any details about his illness.
"I couldn't recall when last I had a blood transfusion," he said. About his treatment in London he added: "Blood transfusions, going to the laboratories and so on and forth."
Buhari, who first led the country from 1983 to 1985 after taking power in a military coup, was elected democratically two years ago. Since then he has traveled to Britain several times to consult doctors.
He is a northern Muslim, while Osinbajo is a lawyer from Nigeria's predominantly Christian south, a political arrangement that reflects Nigeria's broad geographic and religious divisions.
Reflecting his popularity in the north, armed police had to control hundreds of cheering supporters as Buhari's plane landed in Kaduna.
Hundreds also celebrated in Maiduguri, capital of Borno state in the northeast, where the army under Buhari's command has retaken territory previously lost to Boko Haram jihadists.
Osinbajo played an active role in Buhari's absence, chairing cabinet meetings and finishing an economic reform plan needed to secure a World Bank loan to help plug a deficit caused by low oil revenues in Africa's biggest producer.
He also traveled several times to the commercial capital Lagos and the Niger Delta oil hub to calm tensions with militants attacking oil facilities - two regions Buhari had largely ignored.
While Buhari was away, the central bank also devalued the naira for retail customers and investors hope for more. The currency slipped against the dollar on Friday in the non-deliverable forward market, which enables companies or investors to hedge their naira exposure, as devaluation talk gained momentum on expectations that Osinbajo would keep a prominent role.
"The big question must be how many of the changes were carried out with the president's blessing," said Simon Quijano-Evans, emerging markets strategist at Legal & General Investment Management in London.
With Abuja airport closed for six weeks of repairs, fewer visitors will flock to the presidential villa. Buhari may be able to keep a low public profile without raising too much suspicion about his health while Osinbajo can work on the economic file. A trade conference has been shelved.
The transparency over the temporary handover to Osinbajo stands in marked contrast to the secrecy and confusion that surrounded the illness of President Umaru Yar'Adua, who died in 2010 after a long period of medical treatment in Saudi Arabia.
(Reporting by Garba Mohammed, Felix Onuah, Ulf Laessing, Lanre Ola and Karin Strohecker; Writing by Ed Cropley and Ulf Laessing; Editing by Mark Trevelyan)