At least 20 people are believed to have died in a second day of violence in the southern Nigerian city of Onitsha.
Police have struggled to keep a lid on the violence
Groups of armed youths rampaged through the city attacking Muslims, in apparent retaliation for the deaths of Christians in riots in the north.
Eyewitnesses spoke of streets "littered with bodies" as thousands of Muslims were said to be fleeing the city.
Danish cartoon protests in the north led to sectarian clashes which have seen dozens of deaths in four cities.
On Wednesday, groups of Christian Ibo men wielding clubs and machetes rampaged through Onitsha for a second day attacking any members of the Muslim Hausa community they could find, according to witnesses.
One eyewitness said cars driving into the town had been stopped by angry crowds demanding to know if there were any Hausa on board.
Those who were identified were dragged out and taken away.
Onitsha resident Isotonu Achor said he had seen more than 20 people killed.
"Major streets are littered with bodies of people killed today, most of them northerners," he told the Associated Press.
A photographer for Reuters news agency said some the victims had been burnt and some had had their stomachs cut open.
The BBC's Sola Odunfa said when he tried to enter the city, he was confronted by a 200 armed men demanding proof that he was not Hausa.
But he said the violence was "not a question of Christian or Muslim".
"They are just targeting people from the north in anger," he told the BBC's Newshour programme.
The authorities say many Hausas still in the city have sought shelter inside police and army facilities.
Thousands have fled the town, reports said.
Police say they have sent reinforcements into Onitsha, which has been sealed off, and the Anambra state authorities have imposed a curfew on the town.
It is not clear how many people died when the riots began on Tuesday, but reports suggested at least a dozen fatalities, possible more.
There are also reports that the violence has spread to neighbouring towns.
Earlier on Tuesday, the authorities in Bauchi in northern Nigerian imposed a curfew after at least 13 people were killed in a sectarian riot.
The Bauchi riot followed violence over the weekend in the two northern towns of Maiduguri and Katsina, which began as demonstrations against the Danish cartoons satirising the Prophet Muhammad.
Nigeria, roughly divided into a mainly Muslim north and Christian south, has been hit by sectarian violence in the past.
More than 10,000 people died in communal violence across the country in the first few years following Nigeria's return to democratic rule in 1999, but in recent months these clashes have become less frequent.