Nigeria's former national security adviser, Sambo Dasuki, has been arrested for allegedly stealing $2bn (£1.3bn), his representatives say.
Mr Dasuki is accused of awarding phantom contracts to buy 12 helicopters, four fighter jets and ammunition. He denies the allegations.
The equipment was meant for the fight against Boko Haram Islamist militants.
Mr Dasuki was picked up early in the morning by security agents, a PR firm representing him said.
Two weeks ago, President Muhammadu Buhari ordered Mr Dasuki's arrest after he was indicted by a panel investigating the procurement of arm under the last administration of former President Goodluck Jonathan.
PRNigeria said he was picked up by intelligence agents from his home in the capital, Abuja, where he was already under house arrest facing separate charges.
Later on Tuesday, the former governor of Sokoto state, Attahiru Bafarawa and owner of a Lagos-based private TV station, Raymond Dokpesi, were detained separately by Nigeria's Economic and Financial Crimes Commission (EFCC), in connection with the alleged fraud.
Both men, who are political allies of the former president, have denied any involvement in the arms deal.
Their arrest follows those of some of Mr Dasuki's associates by EFCC on Monday.
The anti-corruption body said they included former Minister of State for Finance Bashir Yuguda and the sons of some prominent politicians of the former ruling party over allegations of impropriety in relation to the arm deal.
Earlier, Mr Dasuki said he had not been given a chance to defend himself before the investigative panel and described its recommendation as "politically motivated".
The former army colonel is already facing a trial for allegedly possessing illegal firearms.
He is the first senior official of the former government to be charged under the rule of President Muhammadu Buhari, who took in office in May.
The BBC's Martin Patience in Lagos says Mr Dasuki's arrest will reinforce President Buhari's message that he will not tolerate corruption no matter how senior the official.
The president was elected partly on a promise to clean up Nigeria's notoriously corrupt politics, our correspondent says.
Boko Haram has killed thousands in north-eastern Nigeria in its six-year campaign to create an Islamic state.
- From a royal family, he's the son of the 18th Sultan of Sokoto, spiritual leader of Nigeria's Muslim community
- A career soldier, rose to be a colonel in the army until his retirement
- Reportedly one of the officers who arrested current President Muhammadu Buhari when he was overthrown as military head of state 30 years ago - something he denies
- Became former President Goodluck Jonathan's national security adviser in 2012
- Oversaw the fight against Boko Haram under his presidency
- At the centre of a row over Nigeria's unorthodox arms procurement in 2014, when South Africa seized suitcases packed with millions of dollars of cash at an airport in Johannesburg
- Charged for possessing illegal weapons in August 2015
- Was put under house arrest despite a court order to allow him travel abroad for cancer treatment