Photos of the chaotic scene appeared to show Mr Ellwood leaning over the victim with his two arms extended as paramedics and police crowded around.
Westminster terror attack: Timeline of events
He remained with the victim while an air ambulance landed on nearby Parliament Square.
A source told The Telegraph: “[He] tried to give mouth-to-mouth and stem blood flow from multiple stab wounds to the officer until the chopper and medics arrived.”
Police confirmed there were a number of casualties in the Westminster attack, including police officers.
Commander BJ Harrington said: "We know there are a number of casualties including police officers but at this stage we cannot confirm numbers or the nature of these injuries.
"We received a number of different reports which included a person in the river, a car in collision with pedestrians and a man armed with a knife."
At least 10 patients have been treated on Westminster Bridge and a number of hospitals are on alert, London Ambulance Service said.
Mr Ellwood, an MP for Bournemouth East since 2005, is a Parliamentary Under-Secretary in the Foreign Office.
/The Independent/
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