WASHINGTON — Thousands of Trump supporters are expected to gather Wednesday in the nation’s capital to hear a defeated president and his allies amplify false claims of election fraud during a rally steps from the White House.

Local authorities have enhanced security and warned residents in and around the city to steer clear of potentially violent agitators.

The scene taking shape on the grassy elliptical park just behind the White House is extraordinary. A grand bandstand was erected, huge speakers were attached to tall scaffolding, and thousands of chairs were unfolded on muddy grass ahead of rallies to proclaim victory for a president who lost soundly in November.

President Trump will be leaving office in two weeks, all protests aside.

But he plans to make an appearance on Wednesday at one of the events near the White House that he has promoted relentlessly for weeks as a show of force as he struggles to overturn the legitimate election results.

The events and vitriol had already begun on a rainy Tuesday in Washington, a day before Congress’s formal counting of the Electoral College votes. Some of Mr. Trump’s allies, including the conspiracy theorist and conservative radio host Alex Jones and some associates who recently received a pardon from the president, spoke to hundreds of people who crowded into the city’s Freedom Plaza on Tuesday evening.