Donald Trump made a series of unsubstantiated allegations to claim he is being cheated out of re-election as his Democratic opponent Joe Biden appealed for calm and patience.
The Republican incumbent alleged he was the victim of interference from "phony polls" as well as "big media, big money and big tech" after launching battles to stay in office.
In an extraordinary White House press conference late on Thursday, the president said: "If you count the legal votes I easily won. If you count the illegal votes, they can try to steal the election from us."
The presidential election remains too close to call after polls closed on Tuesday, but former vice-president Mr Biden remains the favourite after winning three key battleground states.
Pennsylvania was expecting to announce its result by the end of the day. If it goes to Mr Biden, so would the White House.