Const. Jana McGuinness says the woman, who suffered a head injury, remains in hospital in serious condition. (CBC)

A 55-year-old woman fell to the ground and hit her head after being stunned by a police Taser Tuesday morning, Vancouver police said.

The woman was shocked after a standoff with officers in East Vancouver, Const. Jana McGuinness said. She remains in serious condition in hospital.

Officers saw the "distraught" woman waving a large folding knife on the sidewalk on East Hastings Street at about 11:30 a.m., McGuinness said.

"Early information indicates that police repeatedly told the woman to drop the knife while they redirected pedestrian traffic away from her," she said in a news release.

"The woman yelled obscenities at the officers challenging them to shoot her."

The woman then approached the officers, who stepped back and drew a Taser but didn't fire any shot.

"The standoff continued until a person in the crowd ran at the woman," McGuinness said. "Police yelled at him to stop but he ignored their directions and grabbed at the woman's arm holding the knife."

The two struggled, but the man was unable to disarm her.

One of the officers fired the Taser at the woman, who fell and hit her head while still clutching the knife, McGuinness said.

Police have not released the woman's name or any other personal information.