"If you count the illegal votes, they can try to steal the election from us," Trump said.
"If you count the votes that came in late, we're looking into them very strongly. A lot of them have come in late."
Trump said there were "now only a few states yet to be decided into the presidential race" and the "voting apparatus are run in all cases by Democrats".
"We were winning in all the key locations by a lot, actually, and then our numbers started miraculously getting whittled away, in secrecy," he said.
With his hopes of holding on to the presidency slipping away as thousands of votes continue to be counted in key battleground states, Trump vowed to fight the outcome in court.
"We think there is going to be a lot of litigation as we have so much evidence, so much proof and it is going to end up perhaps at the highest court in the land," he said.
"We will see. But we think there will be a lot of litigation because we cannot have an election stolen like this and I tell you, I have been talking about this for many months with all of you.
"I have said very strongly that mail-in ballots are going to be a disaster. We are hearing absolute horror stories and we cannot let that happen to the United States of America."
Trump said it was "not a question of who wins, Republican, Democrat, Joe, myself".
"We cannot let that happen to our country.
"We cannot be disgraced by having something like this happen. So it will hopefully be cleared up, hopefully soon but it will probably go through a process, legal process, and as you know I have claimed certain states and he is claiming states and we can both claim the states, but ultimately I have a feeling judges are going to have to rule, but there has been a lot of shenanigans and we cannot stand for that in our country."
Anchors on the US's major news networks immediately checked the President.
"What a sad night for the United States of America to hear their President say that," CNN host Jake Tapper said after the press conference.
"To falsely accuse people of trying to steal the election, to try to attack democracy that way with his feast of falsehoods. Lie after lie after lie."
It came after the President spent Thursday shooting off a series of furious all-caps tweets, writing: "STOP THE COUNT!", "STOP THE FRAUD!" and "ANY VOTE THAT CAME IN AFTER ELECTION DAY WILL NOT BE COUNTED!"
He added, "All of the recent Biden claimed States will be legally challenged by us for Voter Fraud and State Election Fraud. Plenty of proof – just check out the Media. WE WILL WIN! America First!"