The vice-president of Malawi, Cassim Chilumpha, has been arrested on charges of treason, reports say.
President Mutharika tried to dismiss Mr Chilumpha in February
He is accused of holding meetings in which members of his United Democratic Front party conspired to topple the country's president, his lawyer said.
Relations between the two leaders have been strained for some time, correspondents say.
In February a judge stopped the president sacking his deputy for "undermining government integrity".
A police spokesman confirmed the arrest but did not comment on the reason, the Associated Press news agency reported.
UDF spokesman Sam Mpasu said the arrest was a "clear violation of the laws of Malawi".
He said police also arrested another UDF official, Yusuf Matumula, and one of the vice-president's bodyguards.
Mr Chilumpha's lawyer, Fahad Assani, called the move "the continuation of political vindictiveness".
"The meetings that took place there were normal political meetings," AP quoted him as saying. "There is nothing wrong for a leader to meet his officials."
In February 2005 President Bingu wa Mutharika left the UDF to form his own party, accusing his former party of undermining his anti-corruption campaign.
Mr Chilumpha remained in the UDF and the two men have struggled to work together, analysts say.
In February, Mr Mutharika dismissed his vice-president, accusing him of arrogance and abusing his oath of office, but a judge ruled that Mr Chilumpha should stay in office pending a judicial review.