The National Exotics Animal Rescue Service is trying to capture the animal but people are scuppering their efforts by chasing it in 4x4 vehicles.
Officers in Boston said there had been sightings for a "couple of weeks" in the Spilsby area but the bird is at risk of being injured and people should not approach it.
Posting on Facebook, police said: "Experts have been trying to gain her trust by feeding her in the same spot for a while, however, efforts are being scuppered and staff and volunteers are concerned because members of the public have been chasing her in 4x4s.
"If she continues to be agitated in this way, there is a risk of her running into the path of oncoming vehicles causing harm to herself or others."
The National Exotics Animal Rescue Service is on the scene and has called in a specialist team to coordinate the emu's capture, police said.