"If you count the illegal votes, they can try to steal the election from us."
Three big US broadcast networks - ABC, CBS and NBC - cut away from Mr Trump's news conference to correct some of his falsehoods.
“We have to interrupt here, because the president made a number of false statements, including the notion that there has been fraudulent voting,” Lester Holt of NBC Nightly News said.
“There has been no evidence of that,” he added.
Mr Trump also claimed that "big money" and "big tech" companies were interfering in the election.
That appeared to be a reference to Facebook and Twitter, which have been flagging some of Mr Trump’s posts as being misleading.
Donald Trump campaign launches lawsuit in Nevada over vote
Mr Trump's campaign has launched legal action to try and derail the counting of votes in key battleground states such as Pennsylvania, which appear to be swinging towards Joe Biden.
"It is a corrupt system and it makes people corrupt even if they are not by nature but they become corrupt," Mr Trump said during the press conference.
"It is too easy."
Mr Trump also boasted of the "tremendous amount of litigation" he had launched to try and stymie the Democrats.
Mail-in ballots have so far tilted heavily to Democrats.
The big shift to postal ballots this year reflected the desire of voters to avoid risking exposure to COVID-19 in crowded polling stations during a pandemic that has already killed some 235,000 Americans.
Additional reporting by AFP.