The Police have put the number of deaths from gunmen attack at the St. Philips Catholic Church, Ozubulu in Ekwusigo Local Government Area of Anambra on Sunday morning at about 11.
He said the gunmen then went on rampage and "shot at the remaining worshippers numbering".
"The information at the disposal of the police is that the gunman had been hired to kill a particular family person believed to be among the worshippers", he said.
According to the President, there is no religion that does not lay constant emphasis on the sanctity of life, and that "all Nigerians must rise up and speak with one voice against these remorseless evil men".
A witness, who simply identified herself as a choir member, said that the church was dark as there was no light.
Umar added that the injured were taken to the hospital in Nnamdi, the state's second largest city.
The party in a statement condemned the barbarism displayed by the callous attackers who gunned down defenceless people while they worshiped.
The state Governor, Willie Obiano, who visited the scene of the attack has, however, promised that investigations would commence immediately to bring the perpetrators to book.
No one has claimed responsibility for the attack.
"The man after shooting at his targeted victims still went on a shooting spree, killing and wounding other worshippers, " he said.
The gunmen, witnesses further said, first separated a group of worshipers from the rest and started executing them with gunfire.
The rebellion has killed at least 20,000 people and forced some 2.6 million others to flee their homes, sparking a dire humanitarian crisis in the country's northeast.
Security agents were said to have cordoned off the church.
Mr. Emecheta, who is the Managing Director of Anambra Signage and Advertisement Agency, ANSAA, said that the losses from the attack were too much for the people to bear.