Tim Walz, Kamala Harris's running mate, has formally accepted the Democratic vice-presidential nomination at the party convention in Chicago.

In a headline speech in Chicago, the running mate of Kamala Harris touted his small-town upbringing and career as a teacher and coach.

It came after Oprah Winfrey made a surprise appearance on stage, telling the raucous crowd to back Harris in November.

Former US President Bill Clinton, in his 12th consecutive convention speech, hailed Harris as "extraordinary" and took aim at Donald Trump who he said "creates chaos”.