A hospital spokesman in Rivers state said on Wednesday "a total of 15 bodies were brought in" the day before. Twelve wounded people were also taken to the hospital, he added.
Police earlier gave a toll of four dead and four wounded, while Buhari's office announced the death of "several" people.
The crush is the latest in similar incidents in Buhari’s campaign rallies. Last Thursday, two people reportedly lost their lives during a crush at a rally in the northeastern state of Taraba.
On January 21, several people were wounded in the northeastern city of Maiduguri after a platform collapsed during the president’s visit.
According to local media, the stampede happened after Buhari’s campaign speech as the crowd surged towards a partially-locked gate to follow the president, Presstv reported.
"Those from behind were pushing and putting pressure on those in front, leading to some persons falling on the ground and being trampled upon," said a journalist of the Vanguard newspaper.
Rivers state, famous for its oil and gas industry, is a major battlefield between the ruling All Progressives Congress (APC) and the main opposition Peoples Democratic Party (PDP). There are already claims of vote-rigging attempts.
Five APC members were shot to death during clashes on Sunday with PDP followers near the city of Warri, an oil hub in Delta state.
The incidents come ahead of Saturday’s election in which current president Buhari will seek another term.
Buhari took the helm in May 2015 after vowing to improve security and intensify crackdown on endemic corruption, but Africa's top oil producer experienced its first recession in 25 years in 2016, largely caused by low crude prices and militant attacks in the Niger Delta, and its recovery remains fragile.