The city council shall have the care, supervision and control of all public highways, streets, alleys, bridges, subways, causeways and public grounds within the limits of said city, and shall have the power to build and keep in repair bridges, elevated railways, subways, causeways, and to lay out, open, grade, curb, alter, vacate, extend, narrow, widen or straighten any public highway, street, alley and public grounds, and to purchase, hold and convey lands in fee simple, and to take grounds for the site of public buildings and public parks, subject to the assessment of damages as hereinafter provided.