The kidnappers, who wore army uniforms, fired into the air as they stormed the site in three vehicles late on Sunday, ordered everyone in the crowded club to lie on the floor and singled out white males for abduction.
The hostages included two Britons and an Irish national, diplomats said on Monday.
The German foreign ministry said it was checking if one of its citizens was also missing.
Edith Monigha, the bar manager, said: "There was serious shooting in the bar and they left taking away some white men.
"They didn't rob us or ask for anything else, they only wanted the white men."
She said that there were more than seven attackers and that she could not tell how many men were abducted in the raid on Goodfellas nightclub shortly before midnight on Sunday but a colleague said he thought there were four.
Blood stains were visible on the floor of the club on Monday.
Monigha said several people had cut their legs in the scramble to get down.
The gunmen abandoned their cars and escaped in a waiting speedboat after security forces engaged them in a gun battle, police said.
A police spokeswoman in Port Harcourt, capital of Rivers state, said: "the kidnappers ... burnt one of the vehicles they used, maybe to destroy anything that could give them out, and escaped through the waterway."
News of the kidnappings came just after the release of three Filipino gas workers who had been taken hostage 10 days ago.
Later on Monday, two other oil worker hostages, a Moroccan and a Belgian who were kidnapped on August 10, were released, a witness said.
Niger Delta militants demands
have taken on a more political tone
The wave of kidnappings coincides with an upsurge in militant attacks against the oil industry which has cut oil production by 25% in the world's eighth largest exporter since February.
Militancy is fuelled by widespread feelings of injustice in the Niger Delta region where most people live in poverty despite the wealth being pumped from their ancestral lands.
Many abductions are motivated principally by ransom, but some recent incidents have taken on a more political tone, with demands reflecting a growing ethnic nationalism among the Ijaw tribe, which is native to the Niger Delta.
Criminal gangs, sometimes involved in the large-scale theft of crude oil from pipelines, also regularly indulge in kidnapping and extortion, and it is often difficult to distinguish between the two.
Police named one of those seized on Sunday as Briton John Guyan and said he worked for Smith International Inc, a supplier to the oil and gas industry.
The company confirmed one of its expatriate workers had been taken hostage.
Another of those kidnapped was Brayan Fogerty, who works for US oil services company Halliburton, police said.
Separately on Monday, Norway's ambassador in Nigeria said negotiators were close to a deal to free four other foreign workers - two Norwegians and two Ukrainians - kidnapped from an oil services ship off the coast of Nigeria last week.