Abuja, Nigeria (CNN) -- Rural villages held mass burials in northern Nigeria on Sunday, the Red Cross said, raising the possibility that six days of post-election violence have produced heavy casualties.
There is no official death toll, but witnesses say hundreds have died. The government has provided few other details out of concern the violence could spiral.
Shehu Sani, president of the Civil Rights Congress and longtime civil rights campaigner, told CNN there have been 500 burials. CNN could not independently confirm that number. The Civil Rights Congress, based in Kaduna, has in the past been known to ally itself with pro-northern political groups.
The election of President Goodluck Jonathan, who is from the majority-Christian southern part of the country, sparked violence in the Muslim-dominated north last week.
Concerns about ongoing violence led Nigerian election officials last week to delay subsequent gubernatorial elections in two states.
Northern Muslims in some areas charged that the elections were rigged. Armed protesters in the region prowled the streets chanting the name of former military ruler Muhammadu Buhari, the main opposition frontrunner.
The last election four years ago was widely condemned for rampant vote rigging, violence, theft of ballot boxes and intimidation. While the Monday elections were marred by violence, reports of underage voting and other logistical problems, observers considered it an improvement.
World leaders have urged the west African nation to probe allegations of ballot stuffing and unusually high turnout in some areas.
After he was declared the victor, Jonathan warned perpetrators that persistent postelection violence could threaten the stability of the country.
"These acts of mayhem are sad reminders of the events which plunged our country into 30 months of an unfortunate civil war," Jonathan said last week.
Jonathan was referring to a period of unrest in the 1960s that spawned a civil war in Nigeria.
Muslims fearing reprisal attacks in Christian areas have fled to military barracks or neighboring states, the Nigerian Red Cross said.
More than 40,000 people are displaced in the north, according to the aid group. Government officials have declined to release the number of fatalities or injuries over fears it could prompt revenge attacks.
Nigeria is Africa's largest oil producer and its most populous nation with 150 million people.