Nigeria's Supreme Court has annulled April's election of the governor of oil-rich, violence-prone Rivers State.
Celestine Omehia ran for governor at the last minute
The court said current Governor Celestine Omehia had not been the legitimate candidate of the governing People's Democratic Party.
It ruled that the post should go to Rotimi Amaechi, who won the party's primaries, but was dropped on the eve of voting because of graft allegations.
Troops have been deployed in the state capital in case of violence.
It is the third time this month that a Nigerian court has nullified the election of a state governor.
The BBC's Alex Last in Lagos says the governor of Rivers State controls a budget in excess of $1bn, but the region has been wracked by poverty and has become synonymous with violence and kidnappings, so any political instability could have dire consequences.
Our correspondent says that on the streets of the state capital, Port Harcourt, there has been some celebration and some trepidation at the news of the Supreme Court's ruling.
Many of Rivers State's inhabitants live in poverty
Troops have been deployed to try to prevent any outbreak of violence as the many armed gangs in the state have close links to competing local politicians.
The court did not call for fresh elections.
April's poll in Rivers State was marred by massive vote rigging and violence - and was declared a farce by election observers.
Mr Amaechi himself is a controversial figure, our reporter says.
The anti-corruption agency was investigating how he, as the speaker of the state house of assembly, had allegedly amassed a fortune of around $150m.
As governor, he will be immune from prosecution.
On paper, Rivers State in the Niger Delta is the richest region of Nigeria but massive corruption has meant most live in poverty, our reporter says.