31 people were killed on Saturday in  a stampede in southern Nigeria during an overcrowded church charity event where food was being distributed, AFP reports.

Shoes and slippers lay scattered on the ground after the disaster in Port Harcourt city in southern Rivers State when people tried to force their way into the event, say police and witnesses.

A local church organisation was offering food and gifts for the poor at the local Port Harcourt Polo Club when a "mammoth" crowd got out of control, Rivers State police said.

"Unfortunately, the crowd became tumultuous and uncontrollable and all efforts made by the organisers to bring sanity proved abortive hence, the stampede," the police said in a statement.

"A total of 31 persons lost their lives in the stampede."

Police said a criminal investigation was underway.

Representatives of the church could not immediately be contacted for comment.

Nigeria has seen several stampede tragedies over food distribution in recent years, including an aid agency food programme in north Borno State where seven women were trampled to death last year.

Saturday's early morning disaster happened as the opposition People's Democratic Party leaders were gathering in the federal capital Abuja to select their candidate for the 2023 presidential race, including hopeful Rivers State governor Ezenwo Nyesom Wike.

Port Harcourt is the main oil hub in Nigeria, Africa's most populous country and one of the continent's largest petroleum producers.

Despite its oil wealth, as many as four out of 10 Nigerians live below the national poverty level, according to a recent World Bank Report.

The Ukraine crisis has also pushed up the cost of food and fuel across the continent as wheat and gas supplies are impacted with aid agencies warning about worsening food insecurity in Africa.