Whenever Council has ordered the opening, widening or extension of any street, avenue or boulevard, or whenever Council has ordered that proceedings be instituted for the acquisition or enlargement of any park, playground, playfield or other public open space, such action of Council shall not be rescinded until after the matter has been referred back to the Planning Commission for a report and until after a public hearing has been held. Council shall have the power to overrule the recommendations of the Planning Commission by a vote of not less than two-thirds of its entire membership.