Jos was rocked by religious violence last year
At least 20 people died during the fighting, reports say.
Deputy Jos Police Commissioner Haruna John said the clashes were between rival PDP factions.
Observers say the clashes are likely to reinforce fears of wider trouble in nationwide elections just months ahead.
Police imposed a night curfew as panic swept through the tin mining town, which is still recovering from devastating sectarian riots that killed hundreds last year.
Machetes
Clashes flared at a PDP meeting exactly a week after President Obasanjo announced that he would seek re-election early next year.
"I saw 15 bodies in the mortuary, some burned to death and others with machete cuts," said a resident who visited the Jos Teaching Hospital.
Witnesses reported seeing at least five other bodies in the district of Nasarawa, where a row between rival PDP factions flared into fighting that spilled outside the congress venue.
Police said they could not confirm the casualty figures.
"We cannot verify the number of people killed in the clashes because our men are still out on the field," Mr John told Reuters news agency by phone.
"But certainly a number of people were killed and property destroyed."
Local resident Usman Mohammed said:
"There was panic all over the town. Women and children were running all over the place and several fell and were badly wounded."
"I saw two people with arrows stuck in their stomachs. Many, many cars had their windscreens broken," he added.
Appeals for calm
Authorities, fearing a rapid degeneration of the situation as was the case during clashes between Muslims and Christians last September, quickly deployed soldiers to back up the police.
"Police have just ordered a night curfew over the affected district of Nasarawa. The police are broadcasting appeals for calm over the radio," said another resident contacted by phone.
Police were quick to play down any religious or ethnic link this time, apparently for fear of reprisal attacks.
"We are treating it for now as a political issue because so far we have nothing to prove that it was ethnic or religious," said Mr John.
"We have been able with the help of the military, to bring the situation back to normal."
Many Nigerians are bracing for renewed tension as Africa's most populous country begins a period of intense political activity.
Local elections are scheduled for 10 August, followed by presidential and parliamentary polls expected in April 2003.