"See you in Washington, DC, on January 6th. Don't miss it," he wrote on Sunday.
Fears of violence
The rally raised fears of fresh violence after the previous pro-Trump protest that included the Proud Boys on 12 December saw several people stabbed and dozens arrested.
Mr Trump appears to hope that the protesters could pressure Congress to reject the final count of state-based electors and reverse his election loss.
"We the People must take to the US Capitol lawn and steps and tell Congress #DoNotCertify," StopTheSteal declared online.
"Congress cannot certify this fraudulent Electoral College," they said.
On 6 January Vice President Mike Pence is to lead Congress in certifying the Electoral College votes submitted by each state, which represent the results of the popular vote.
In the joint session of the House of Representatives and the Senate, Mr Pence is to open and read the certificates reporting elector tallies from each state, and then declare the victor.
That process should be, as it almost always is, a formality.
Mr Biden captured 306 electors while Mr Trump only won 232.
Mr Trump's campaign has lost dozens of court challenges in several contested states, with judge after judge saying they showed no evidence of any significant fraud.
But the session can be stalled if lawmakers from both houses submit formal objections to any state's report.
Alabama Representative Mo Brooks told Fox News on Monday that he has the support of "dozens" of lawmakers to formally challenge the counts.
Meanwhile, Mr Trump and supporters have pressured Mr Pence to unilaterally reject some pro-Biden states' elector certificates, a power legal experts say Mr Pence does not have.
Republican Congressman Adam Kinzinger on Sunday said it was all an effort by "certain members of Congress and the president" to raise money.
"It is a scam and it is going to disappoint the people that believe this election was stolen," he told CNN.