Some passengers were paid to take part in a buzz-generating video which showed Norway's prime minister undercover as a taxi driver probing voters' concerns, his party admitted Monday.

Just one month before Norway's legislative elections that Prime Minister Jens Stoltenbeg's centre-left coalition appears likely to lose, a video was on Sunday released showing him wearing a taxi driver's uniform and sunglasses, sitting at the wheel as he drives passengers around Oslo.

The video quickly generated a lot of buzz on social networks and in the media.

In power since 2005, Stoltenberg explains at the beginning that he wants to get close to voters to hear their concerns.

But on Monday, tabloid Verdens Gang (VG) revealed that five of the 14 passengers filmed with hidden cameras fitted in the cab were in fact chosen during a casting call.

"They're five ordinary people who were asked if they wanted to take part in a video for the Labour Party and who knew nothing else, except that they were going to be picked up in a taxi," party spokeswoman Pia Gulbrandsen said.

"Their spontaneity was real when they realised that the driver was the prime minister," she said.

Each of the five received 500 kroner (65 euros, $85) "as a thank you", the Labour Party said.

None of the passengers had to pay their fares either, since Stoltenberg does not have a permit to drive a taxi.

The owner of the public relations firm that made the video, Kjetil Try -- a friend of the prime minister's -- told VG the casting call was necessary to ensure that passengers were available at the right time and that they represented a broad diversity.

The video was on the whole well-received for its humorous aspects, though several commentators remarked that it focused more on the passengers' surprise than on the voter issues Stoltenberg said he wanted to explore.

Recent opinion polls show Stoltenberg lagging far behind the right wing ahead of the September 9 election, and the success of the video was not expected to be enough to turn things around.