(updates with details, curfew)

WARRI, Nigeria, Feb 3 (Reuters) - A dawn-to-dusk curfew was imposed in the southern oil city of Warri on Monday, following a third night of ethnic violence which brought the total number of deaths to over 20, including members of the security forces.

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Youths went on the rampage overnight in Warri, in the oil-rich Niger Delta, killing police and setting dozens of homes and offices ablaze, witnesses said.

Delta State spokesman Abel Oshevire said a curfew lasting from 7 p.m. (1800 GMT) to 6 a.m. would be in effect in Warri until calm returns.

The clashes are the worst in Warri since 1999, when around 20 people were killed in political skirmishing between ethnic Ijaws and Itsekiris.

The latest violence has pitted ethnic Urhobos against Itsekiris in a long-standing dispute over the location of government offices and amenities.

A Reuters reporter saw 18 bodies in the streets on Saturday, and security forces said soldiers and police were among the victims.

The town in southern Delta State was calm but tense during daylight hours on Monday.

Troops and police patrolled the streets, setting up checkpoints on key junctions. Many civilians were seen packing up their possessions in preparation to flee to safer areas.

Banks and shops were boarded up, the witnesses said.

Sunny Areh, spokesman for the state governor, said the latest clashes were sparked by a row over the delineation of local electoral wards. He was unable to provide an official casualty toll.

"All I have in life: money, property is gone. My home has been burned down and my caravan where I sell second-hand electronics has been burned...I am finished," said Ifeanye Onuorah.

Onuorah lived in Okumagba Estate, an Urhobo area of Warri where many houses were razed on Sunday, allegedly by Itsekiri youths.

A hotel complex of about 20 bungalows was burned down in Sunday night's violence and a police station was gutted by fire. Police said that two of their colleagues were killed in the rioting overnight.

Lieutenant Colonel Gar Dogo, commander of Warri's army battalion, said on Saturday some soldiers had been killed, without specifying how many.

Over 10,000 people have died in ethnic, political and religious violence in Nigeria since the country's return to democracy after 15 years of military rule in 1999.

Worries over violence in Africa's most populous country are increasing ahead of presidential and local elections set for April.