Federal authorities are building a "non-scalable" fence around the White House ahead of Tuesday's U.S. Presidential election that many fear may lead to civil unrest or worse, reports said.

Tensions are high across Washington, D.C., and the rest of the country ahead of what is expected to be a partisan election that could take days to confirm a result.

NBC News and CNN said officials are erecting a fence similar to the one installed in June as riots jolted the country's capital following the death of George Floyd at the hands of Minneapolis police.

Additional fencing has already been installed around Lafayette Square, across Pennsylvania Avenue from the White House. The perimeter will form a square fence around the White House, on 15th Street, 17th Street, Constitution Avenue and H Street. Some 250 National Guards have been placed on standby, NBC News reported.

Tuesday's election looks to be more intense than usual with far-right groups warning they will come to the polling stations with weapons. Law-enforcement units in several cities are planning for the likelihood of street clashes.

The National Security Council, the Federal Emergency Management Agency and Office of Personnel Management including the military have met more than 60 times since January this year in preparation for any incident.

Washington, D.C., residents were bracing for disorder regardless of the poll outcome. Shop owners boarded up windows against looting while George Washington University directed students and faculty to stock at least a week's food, medicine and other necessities. Business and government centers in Minneapolis and San Francisco were boarded up as were some in New York.

Threat-monitoring group International Crisis Group released a 30-page document last week saying that Americans faced an "unfamiliar danger" in the coming days.