Malawian police and anti-graft agents have raided the house of a High Court judge hours after he ruled against the government in a row over the budget.
Mr Mutharika's row with his former party could paralyse the country
Joseph Mwanyungwe issued an injunction allowing the opposition-dominated parliament to continue stalling a debate on the budget suspended in July.
Opposition MPs are refusing to discuss it unless MPs who switched to the president's party are expelled.
Correspondents say the legal battle could topple the government.
Hundreds of students have been protesting outside parliament demanding that the budget is passed to provide essential services to Malawi's 12m people.
Following news of the injunction, the demonstrators threw stones at the building and barricaded the MPs inside for more than five hours.
Anti-corruption bureau officials confirmed the raid on Monday night, but have given no further details about why it was carried out.
Mr Mwanyungwe, who was not at home at the time of the raid, told AFP news agency he was not informed what the police were looking for.
"I don't know whether I am under arrest or not and for what reason," he said.
His lawyer Fahad Assani told Reuters news agency that they suspected the raid was linked to "the injunction he gave to the opposition stopping parliament from meeting".
The current political impasse began in June, when the Supreme Court ruled that the speaker of parliament can expel MPs who switch parties.
Most members of President Bingu wa Mutharika's party were elected on the ticket of the former ruling party, the United Democratic Front (UDF).
Mr Mutharika also won elections for the UDF, but left to set up the Democratic Progressive Party (DPP) - accusing UDF officials of blocking his anti-corruption drive.
Analysts say should the speaker expel the floor-crossing MPs, it could take six months to organise all the by-elections which would ensue.