Jonathan, seeking re-election as president, is the first head of state from the Niger Delta [EPA]

As many as 10 people are though to have been killed in a stampede at an election campaign rally for Goodluck Jonathan, Nigeria's president, in the southern Nigerian city of Port Harcourt.

Thousands of supporters gathered in a stadium in the oil city on Saturday to hear Jonathan speak, with hundreds more crowding outside.

Witnesses said the stampede began after a policeman fired into the air as people left the venue.

"I can see the bodies of three women," Tonye Ben, a witness, was reported by the Reuters news agency as saying.

Other witnesses said as many as 10 people had been trampled to death.

Members of the security forces lifted those who had collapsed into pick-up trucks to be ferried to hospital.

Rita Inoma-Abbey, a police spokeswoman, confirmed that several people had collapsed and been taken to hospital but could not immediately give a death toll.

Jonathan said he was shocked at the loss of life and ordered an immediate investigation.

"I am sad and heavily weighed down by this incident. It is sad, unfortunate and regrettable. I mourn with those who mourn tonight," he said in a statement.

Nigeria is set to hold presidential, parliamentary and state elections in April.

Tha rally by Jonathan's ruling People's Democratic Party came at the end of a week-long tour of the country's six main regions.

Jonathan is the first head of state from the Niger Delta, of which Port Harcourt is the main city, and is considered the front-runner in the presidential race.