* Rights groups say government stifling dissent

* Vote not expected to challenge ruling EPRDF party

* Government lifts economy, critics say at cost of freedoms

By Aaron Maasho

ADDIS ABABA, March 11 (Reuters) - Ethiopia's sole opposition member of parliament said on Wednesday he would not run in the May election and his party would not field candidates because of state meddling in the party's leadership, casting a new shadow over the vote.

Rights groups have criticised the government for stifling dissent in the media and detaining opponents in the run-up to the election. They have also accused the authorities of abuses in previous polls. The government has denied all the charges.

In 2005, 174 opposition politicians won seats in the 547-seat parliament but many did not take them up after pronouncing the vote rigged. In 2010, Girma Seifu, deputy president of the Unity for Democracy and Justice (UDJ), was the sole opponent to win.

"I will not run for the upcoming election. It is because the government forced us not to run," Girma, 48, told Reuters. "All of the UDJ nominees are not running for this election."

The Ethiopian People's Revolutionary Democratic Front (EPRDF), a coalition of four regional parties, has been in power in the Horn of Africa country for about a quarter of a century since communist rulers were overthrown. Analysts do not expect that to change in the May vote.

Analysts said the UDJ was one of only two parties that have been vocal opponents of the government. The other is the Blue party, which is also expected to field candidates.

Girma said a handful of people in the UDJ had challenged the choice of party president last year but that the election commission exaggerated the dispute so it could step in to put "an individual who was not part of the party" into the top job.

Government spokesman Shimeles Kemal denied this, saying the panel had acted to resolve a row and ruled in favour of a rival camp to Girma's. He said Girma could challenge this in court.

"The electoral board is entitled to settle such disputes," the spokesman said. "Girma's group is aggrieved by this decision and they have started to demonise and incriminate the board by coming up with this conspiracy theory."

Ethiopia has many registered parties, but Girma said they did not represent a genuine challenge to the government. "Most of them are established in the name of opposition, but they are not real opposition," he said.

He said he had filed a case seeking to reclaim the UDJ name.

The EPRDF, previously led by the late rebel commander and premier Meles Zenawi and now by Prime Minister Hailemariam Desalegn, has been widely credited for rebuilding Ethiopia's shattered economy after years of famine and conflict.

But critics accuse the government of using authoritarian tactics, such as detaining bloggers and silencing opponents.

The government says it guarantees free speech, only detains those who break the law and runs fair elections in which opponents are free to stand.

(Additional reporting and writing by Edmund Blair; Editing by Mark Heinrich)