The third-party candidate Peter Obi said he would challenge the outcome of Nigeria’s fiercely fought presidential elections after official results awarded victory to the ruling party’s candidate, Bola Tinubu. “We will explore all legal and peaceful options to reclaim our mandate. We won the election and we will prove it to Nigerians,” Obi, the Labour party candidate, told reporters on Thursday in the capital, Abuja. Tinubu, a former Lagos governor, is set to succeed President Muhammadu Buhari, who will step down in May after two terms. He will face immense security and economic challenges in Africa’s most populous country. Almost 25 million people cast a ballot on Saturday in a vote that was largely peaceful but marked by long delays and the slow arrival of online results, angering voters and opposition parties who allege massive vote-rigging. Obi, 61, obtained the third largest number of votes, at 6.1m, according to results announced on Wednesday – a significant feat for an outsider in a country where two establishment parties dominate. Tinubu, of the ruling All Progressives Congress, was declared winner with 8.8m votes and the required number of votes across two-thirds of Nigeria’s states. The former vice-president Atiku Abubakar, 76, of the opposition People’s Democratic party, obtained the second largest number of overall votes, at 6.9m. Abubakar has not yet officially commented on the outcome of the election after losing his sixth bid for the presidency. Candidates who want to submit legal challenges have 21 days after the announcement of results to bring their case to the courts. Tinubu on Wednesday called on his rivals and their supporters to “join hands” with him, urging them to “come in so that we may begin the task of rebuilding our national home together”. Nigerian elections have often been marked by fraud allegations and violence. In an attempt to address some of those concerns, the independent national electoral commission (INEC) this year introduced biometric voter identification for the first time at the national level as well as IReV, a central online database for uploading results. But some voters and opposition parties said failures in the system when uploading tallies allowed for ballot manipulation and disparities in the results from the manual counts at local polling stations. International observers, including from the EU, also noted logistical problems, disfranchised voters and a lack of transparency by the INEC. An umbrella group of Nigerian civil society organisations and observers said the process “cannot be considered to have been credible”. “Given the lack of transparency, particularly in the result collation process, there can be no confidence in the results of these elections,” the Situation Room coalition said on Wednesday. Glitches with the new technology caused huge delays and queues, discouraging some people from voting. With the number of registered voters at 93.4 million, the INEC said turnout was just over 27%, less than in the previous 2019 election. Obi, who for many, especially young Nigerians, represented hope for change, said the election “will go down as one of the most controversial elections ever conducted in Nigeria”. “The good and hardworking people of Nigeria have again been robbed by our supposed leaders whom they trusted.” The INEC has dismissed claims that the process was not free and fair. Tinubu, a longtime political power broker, campaigned on his experience as Lagos governor from 1999 to 2007, charging ahead with the slogan “It’s my turn” to govern Africa’s largest economy. But critics have questioned his health, accusations of past graft, and ties to Buhari, who was criticised for failing in his promise to make Nigeria safer. The country is facing serious security threats, from a grinding Islamist insurgency in the north-east, bandit militias in the north-west and separatist tensions in the south-east. The government has struggled to address a flagging economy, inflation and high unemployment. Although Nigeria is Africa’s largest oil producer, it imports nearly all its fuel because of a lack of refining capacity, and spends billions of dollars subsidising petrol each year – an unsustainable cost that Tinubu has promised to scrap.