Democratic Senator Elizabeth Warren made her bid for the White House official on Saturday in the working-class Massachusetts city of Lawrence.

Warren delivered a populist call for change at her presidential kickoff, decrying a "middle-class squeeze" that has left Americans crunched with "too little accountability for the rich, too little opportunity for everyone else".

She and her backers hope that message can distinguish her in a crowded Democratic field and help her move past the controversy surrounding her past claims to Native American heritage.