Shoes and belongings are left behind during a stampede in Port Harcourt in this video frame grab taken February 12, 2011. REUTERS

At least 11 people were trampled to death on Saturday in a stampede at an election campaign rally for Nigerian President Goodluck Jonathan in the southern oil city of Port Harcourt.

Thousands of ruling party supporters crowded into a sports stadium to hear Jonathan speak and hundreds more gathered around the gates. There was a stampede after a policeman fired into the air to try to disperse the crush as people left, witnesses said.

I can see the bodies of three women, one witness, Tonye Ben, told Reuters as he tried to escape the pandemonium.

The health commissioner of Rivers state, of which Port Harcourt is the capital, told reporters 11 people were killed.

Members of the security forces lifted those who had collapsed into pick-up trucks to be ferried to hospital. Discarded shoes and pieces of clothing littered the ground where people had struggled to free themselves.

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.

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

There was a huge crowd, the place was completely full, Ken Saro-Wiwa, Jonathan's special assistant on international affairs, told Reuters.

As the convoys were leaving, more people were trying to come in and there was a stampede when the gates were opened ... It is a sad end to what had been a great day, he said.

The People's Democratic Party (PDP) rally in Port Harcourt came at the end of a week-long tour by Jonathan of the six main regions in Africa's most populous nation ahead of presidential, parliamentary and state governorship elections in April.

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.

Political rallies in Nigeria often draw crowds beyond the party faithful, many attracted by the promise of free handouts such as campaign T-shirts and baseball caps, leaving the police and security forces struggling to maintain order.