PORT HARCOURT, Nigeria (AP) — A civil rights group says at least 500 people died in rioting that followed Nigeria’s presidential election.

The Civil Rights Congress of Nigeria said Sunday that the worst hit area was Zonkwa, a town in rural Kaduna state, where more than 300 people died in rioting. The group says other killings took place in Zangon Kataf, Kafanchan and the state’s capital of Kaduna.

The group has called for a federal investigation into the killings.

The violence erupted as election results showed Christian President Goodluck Jonathan had won the vote. Many in the predominantly Muslim north of Africa’s most populous nation felt the next president should have been from their region because a Muslim president died last year before he could complete his term.

THIS IS A BREAKING NEWS UPDATE. Check back soon for further information. AP’s earlier story is below.

BAUCHI, Nigeria (AP) — At least 11 recent college graduates who helped run polling stations as part of the country’s national youth service corps have been killed in postelection violence in northern Nigeria and other female poll workers have been raped, police said Sunday.

Authorities have sought to reassure members of the National Youth Service Corps that it will be safe for them to take part in Tuesday’s gubernatorial elections being held in 29 states.

Police have arrested 68 suspects in connection with the deadly riots that were sparked by the April 16 presidential election, Bauchi Police Commissioner Amana John Abakasanga told The Associated Press.

The violence erupted as election results showed Christian President Goodluck Jonathan had won the vote. Many here in predominantly Muslim north of Africa’s most populous nation felt the next president should have been from their region because a Muslim president died last year before he could complete his term.

Retaliatory violence by Christians soon followed, and officials say more than 40,000 people have now fled their homes. Authorities are fearful that releasing any official death toll will only prompt more fighting, but witnesses believe hundreds have been killed across the north.

Nigeria’s National Youth Service Corps is a mandatory yearlong assignment for all Nigerians who graduate from university before the age of 30. Most serve as teachers, but the April national elections have brought extra responsibilities — and danger — to their work.

Ironically the founders of the program created it in 1973 to promote national unity in a country with more than 150 ethnic groups and to help reconcile Nigerians after a 31-month civil war claimed as many as 1 million lives. By encouraging young graduates to explore new parts of the country, the goal was to dispel negative stereotypes.

At least four of the young poll workers were killed in Bauchi last week when an angry mob locked them inside their youth hostel and set it ablaze. The vote for governor has been delayed in Bauchi state until Thursday because of security concerns there.