Police fired tear gas to break up a crowd protesting against the arrests in the main city, Blantyre.
Among those arrested are government minister Goodall Gondwe and ex-minister Peter Mutharika.
They are accused of plotting a coup to try and prevent President Joyce Banda from taking power last year.
‘Vehicles smashed’
Her predecessor, Bingu wa Mutharika, unexpectedly died of a heart attack last April.
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She [President Banda] goes on public podia preaching forgiveness, even quoting biblical verses – is there forgiveness here?”
Callista MutharikaEx-first lady
“I can conform that following the release of a report into the death of President Mutharika, the law enforcement agencies in Malawi are of the strong view that certain criminal offences were committed by certain individuals and as a result arrests have been effected,” Information Minister Moses Kunkuyu told the BBC.
Eight former government ministers have been arrested, including ex-Health Minister Jean Kalirani and Presidential Affairs Minister Nicholas Dausi, says the BBC’s Raphael Tenthani in Blantyre.
The report, presented to Ms Banda last Wednesday following an official inquiry, alleged that government ministers and other senior officials held a number of secret meetings aimed at preventing Ms Banda – then vice-president – from assuming the presidency in accordance with the constitution, our reporter says.
According to the report, Peter Mutharika, a brother of the dead president and a former foreign minister, and Mr Gondwe, the current economic planning minister, had suggested to army commander Gen Henry Odillo that the military “just take over”, our correspondent says.
However, the report quotes Gen Odillo as saying “he was uncomfortable with the suggestion for it was not provided for in the constitution”, our correspondent adds.
Peter Mutharika is the presidential candidate of the former ruling Democratic Progressive Party (DPP) in elections due next year.
Police fired teargas at around 500 people protesting against the arrests outside police headquarters in Blantyre, while another group, estimated to be in the thousands, blocked the main highway to the capital, Lilongwe, with boulders and branches, AFP news agency reports.
Several vehicles were smashed and policemen assaulted during the protests, our correspondent says.
Police spokesman Nicholas Gondwa said no arrests had been made.
Former First Lady Callista Mutharika, who went to the police station where her brother-in-law was being held, told agitated DPP supporters that she was disappointed with President Banda.
“We are told that women leaders are better because women are empathetic – is this what we are seeing now? She [Ms Banda] goes on public podia preaching forgiveness, even quoting biblical verses – is there forgiveness here?” she said.
The former first lady was whisked away to safety after police broke up her impromptu press conference with tear gas, our reporter says.