Reaction to the comments was swift, with Piers Morgan calling it "an unprecedented assault on American democracy".
During his often rambling speech, Trump told supporters that he was on track to secure reelection before the counting of votes stopped "all of a sudden".
"We were getting ready for a big celebration. We were winning everything and all of a sudden it was called off. We were all set to get outside and celebrate something so beautiful, so good, such a success.
"This is a fraud on the American public. This is an embarrassment to our country. We were getting ready to win this election. Frankly, we did win this election. So our goal now is to ensure the integrity, for the good of the nation, we want the law to be used in a proper manner.
"So we'll be going to the Supreme Court. We want all voting to stop. We don't want them to find any ballots at 4am in the morning and add them to the list. It's a very sad moment. To me, this is a very sad moment.
"As far as I'm concerned, we already have won it," Trump said.
Trump argued he was too far ahead in the electoral count to be defeated.
"We were winning everything and all of a sudden it was all called off," he said.
"This is a record. There's never been anything like it. We won states we weren't expected to win. Florida — we won it by a lot. We won the great state of Ohio. We won Texas by 700,000 votes.
"It's also clear that we have won Georgia. We're up by 117,000 votes with only 7 per cent left. They can't catch us. Likewise, we've clearly won North Carolina.
"We're winning Pennsylvania by a tremendous amount of votes. We're up 690,000 votes in Pennsylvania. These aren't even close."
Trump earlier has accused his rivals of trying to "steal" the election as his presidency hangs in the balance, which Twitter immediately flagged.
Prior to Trump's address, Democratic challenger Joe Biden gave a speech to supporters in which he revealed his confidence in winning the election.
"Keep the faith, guys. We are going to win this. Thank you, thank you, thank you. Your patience is great," he said.