The state, which Democratic candidate Joe Biden is currently leading in, is expected to decide the overall result of the election by giving the former vice president enough electoral votes to reach the 270 threshold for victory.
Mr Trump has repeatedly attempted to question the validity of vote counts in key swing states and made unfounded claims that the election is being “stolen” from him.
“Tens of thousands of votes were illegally received after 8pm on Tuesday, Election Day, totally and easily changing the results in Pennsylvania and certain other razor thin states,” the president wrote on Twitter on Saturday.
“As a separate matter, hundreds of thousands of Votes were illegally not allowed to be OBSERVED…”
He added: “This would ALSO change the Election result in numerous States, including Pennsylvania, which everyone thought was easily won on Election Night, only to see a massive lead disappear, without anyone being allowed to OBSERVE, for long intervals of time, what the [sic] happened…”
Mr Trump went on to claim in all-caps, again without evidence, that “BAD THINGS HAPPENED INSIDE” vote counting facilities in the swing state.
Although the president led in Pennsylvania on election night, his lead over Mr Biden has been cut down and overturned following further counting of mail-in ballots.
The change in the result over time has been caused in part by Republican voters disproportionately voting in person on election day, while Democrats were more likely to vote by mail due to safety concerns amid the coronavirus pandemic.
The fact that Mr Trump was leading early in the counting process and is now behind is not evidence of fraud or manipulation and the scenario was widely-discussed as a potential outcome by political analysts before the election due to the unprecedented increase in mail-in voting this year.
Mr Biden was leading in Pennsylvania by 28,836 votes as of Saturday morning, with thousands more ballots to be counted which are expected to be favourable for the Democratic candidate.
All three of the president’s tweets have been flagged by Twitter as containing information that is “disputed and might be misleading about an election or other civic process”.
Election officials in the battleground states of Arizona, Georgia, Michigan, Pennsylvania and Nevada - both Republican and Democrat - have all said they have seen no widespread voting irregularities or major instances of fraud or illegal activity.
Even Mr Trump's own administration has pushed back against claims of widespread voter fraud and illegal voting, without mentioning the president as the source of many of these allegations.
The Cybersecurity and Infrastructure Security Agency, which oversees US election security, noted this week that local election offices have detection measures which “make it highly difficult to commit fraud through counterfeit ballots.”
Additional reporting by AP
Register for free to continue reading
Registration is a free and easy way to support our truly independent journalism
By registering, you will also enjoy limited access to Premium articles, exclusive newsletters, commenting, and virtual events with our leading journalists
Already have an account? sign in
Join our new commenting forum
Join thought-provoking conversations, follow other Independent readers and see their replies