Law enforcement officials say the intense public interest and historic nature of the vote could lead to violent outbreaks if people are unhappy with the results, encounter problems casting their ballots or suspect voting irregularities.
Police departments say they cannot rule out disorder and are mobilising extra forces and putting SWAT teams on standby.
In Oakland, near San Francisco, police will have tactical squads, SWAT teams and officers trained in riot control on standby.
"We always try to prepare for the worst," said Oakland police department spokesman Jeff Thomason.
"This election is going to mark in history a change in the presidency: you're going to have a woman in the presidency or an African American as president. I think everybody around here is voting for Obama, so if he gets in the White House everybody's going to be happy.
"But we'll have our SWAT teams on standby and traffic teams here, so if something goes off we'll organise and take care of the problem."
There have also been internet rumours about plans for protests or civil disobedience by supporters of Democratic candidate Barack Obama if he is beaten by Republican rival John McCain on November 4.