Virginia Etiaba has been sworn in as Nigeria's first female governor after Anambra state legislature impeached Peter Obi for gross misconduct.
She had been deputy governor in the south-eastern state and initially refused to take the position.
Mr Obi said he would not leave office and civil rights groups have criticised the whole process as unconstitutional.
A BBC correspondent says the battle could get more volatile if supporters of the two sides take to the streets.
Both Mrs Etiaba and Mr Obi, from the opposition All Progressives Grand Alliance (APGA), only occupied their posts in March this year.
They took power after a court ruled that Mr Obi's predecessor had rigged the election in 2003.
Mr Obi told the BBC that he does not feel disappointed by Mrs Etiaba's decision to assume the office of governor as she was "just following the rule of the law".
He said that he would fight the impeachment in court as he "believes in the constitution".
The BBC's Abdulsalam Ahmed in Anambra's state capital, Awka, said 64-year-old Mrs Etiaba looked shocked after the impeachment on Thursday.
"The governor is my boss and he remains my boss," she said at the time.
But on Friday morning she was sworn in by the state's chief judge.
In a brief speech, she said she had accepted the position because of the "peculiarity of the situation".
This is the fifth state to start impeachment proceedings against a governor this year.
Ex-Governor Ngige was sacked earlier this year
Some say the recent impeachments are part of the fight against corruption.
But critics say it is often more about rival politicians trying to take charge of the state so they can control the elections due early next year.
Chris Ngige, from the governing People's Democratic Party, had been Anambra's governor for three years, but had fallen out with senior party figures soon after the election.
He said it was because he refused to loot public money to pay them back for their support.
Since Mr Obi took office in March, he has been locked in a power struggle with one of the president's closest aides, Andy Uba, who wants to win the governorship in next year's elections.
Analysts say the impeachment process could only help this ambition.