US Vice President and Democratic presidential candidate Kamala Harris made a surprise appearance at the party's national convention in Chicago on Monday night.

"Let us fight for the ideals we hold dear and let us always remember, when we fight we win!" Harris said in brief remarks that drew roars from the crowd.

“This is going to be a great week and I want to kick us off by celebrating our incredible president Joe Biden,” she said, before turning to Biden directly and thanking him for his leadership.

“Joe, thank you for your historic leadership, for your lifetime of service to our nation and for all you continue to do,” she said, adding, “We are forever grateful to you.”

Harris went on to address the convention, urging unity among her supporters.

“And looking out at everyone tonight, I see the beauty of our great nation. People from every corner of our country and ever walk of life are here, united by our shared vision for our country,” she said, adding, “And this November, we will come together and declare with one voice as one people we are moving forward.”

Harris is slated to speak to the convention again on Thursday, when she will formally accept the party’s nomination for president.