A judge said there was no evidence of Timipre Sylva's election
Extra troops have been deployed on the streets of the capital of Nigeria's oil-rich Bayelsa State after a court quashed the election of its governor.
During his time in office, deposed Governor Timipre Sylva signed a peace deal with oil militants in the state.
It is the 10th such ruling since polls a year ago. Fresh Bayelsa elections were ordered to be held within 90 days
But Bayelsa MPs refused to accept the ruling that the speaker be sworn in as governor and immediately impeached him.
The BBC's Abudullahi Kaura in the Niger Delta says the feud is linked to power struggle between Mr Sylva, of the ruling People's Democratic Party, and Goodluck Jonathan, who was Bayelsa's governor before becoming vice-president.
Bayelsa was one of the main bases for militants fighting for greater control of the Niger Delta's oil wealth.
A peace deal was struck in December - at the instigation of Mr Sylva who travelled to militant strongholds to urge reconciliation, our reporter says.
Other states in the Niger Delta are still struggling with a wave of violence including kidnappings.
Judge Saka Ibiyeye said there was no evidence that the 14 April gubernatorial election actually took place and that election material had been hijacked.
He ordered a fresh election in the state within 90 days.
But it is not clear whether Worinepere Seibaragu, impeached on Tuesday as Bayelsa's House of Assembly speaker, will be sworn in as governor.
Correspondents say the streets of Bayelsa's capital city, Yenagoa, were largely empty as soldiers deployed in armoured personnel carriers after the court ruling.
Pockets of opposition supporters celebrated in some parts of the town.
Nigeria's April 2007 elections were meant to be a landmark in the country, marking the first transfer of power from one civilian leader to another.
But the polls were marred by widespread ballot-stuffing, vote-rigging and intimidation.
European Union observers said the results were not credible.
Hundreds of results are being contested at tribunals, which have cancelled dozens of state and federal legislative results.
Nine other gubernatorial results have also been quashed.
The country's anti-corruption agency has also charged eight out of 36 state governors from Nigeria's previous administration with corruption since they stepped down last year, losing their immunity from prosecution.