Trump has said repeatedly without evidence that mail-in votes are prone to fraud, though election experts say that is rare in US elections
Philadelphia police said on Friday they are investigating an alleged plot to attack the city's Pennsylvania Convention Center, where votes from the hotly contested presidential election are being counted.
Local police received a tip about a Hummer with armed people driving up from Virginia with plans to attack the convention center, a police representative said.
Police took at least one man into custody and seized a weapon as well as the Hummer about which they had received a tip. No injuries were reported and no further details about the alleged plot were disclosed.
The news was reported earlier by Action News, an ABC affiliate. Video footage broadcast by the outlet showed a number of police officials at the scene.
Earlier on Thursday, supporters of both US President Donald Trump and his Democratic challenger Joe Biden held rallies in Philadelphia as election staffers slowly counted thousands of mail-in ballots that could decide Pennsylvania's crucial 20 Electoral College votes.
Trump activists waved flags and carried signs saying: "Vote stops on Election Day" and "Sorry, polls are closed" as Biden supporters danced to music behind a barricade across the street earlier in the day.
A state appellate court ruled on Thursday that more Republican observers could enter the building in Philadelphia where poll workers were counting ballots.
The US Postal Service (USPS) said about 1,700 ballots had been identified in Pennsylvania at processing facilities during two sweeps late on Thursday and were in the process of being delivered to election officials.
Trump has said repeatedly without evidence that mail-in votes are prone to fraud, although election experts say that is rare in US elections.