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.
He said Oakland was prepared to deal with unrest as Oakland Raiders fans rioted in 2003 following their Super Bowl loss.
Other cities that have experienced unrest include Detroit, Chicago and Philadelphia and are also planning to deploy extra officers on election day.
James Carville, a strategist for former President Bill Clinton and advisor to his wife Hillary’s 2008 presidential campaign, hinted Democrat supporters could be angry if Mr Obama lost, given his lead in the polls.
"If Obama goes in and he has a consistent five-point lead and loses the election, it would be very, very, very dramatic out there," he told CNN.
James Tate, of Detroit’s police department, which dealt with violent celebrations after the Detroit Tigers won the baseball World Series in 1984, told congressional newspaper The Hill that problems could flare whichever candidate wins.
"Either party will make history and we want to prepare for celebrations that will be on a larger scale than for our sports teams," he said. "The worst-case scenario could be a situation that requires law enforcement."
In Chicago, where Mr Obama will hold a rally on November 4, the police department has been meeting to discuss security plans for the night. Law enforcement departments in Philadelphia and Cincinnati are also making preparations in case of problems.
Commentators point to the surge in voter registration and large turnout in the primaries as reasons why there could be problems on election day, questioning whether the system will be able to handle so many extra voters.
Election officials in Virginia are stepping up security at polling booths amid concerns over long waits and issues such as voter registration and identity verification.
Despite efforts to improve voting systems after the problems of 2000 and 2004, the Pew Research Centre has warned high turnout could again cause problems such as lengthy delays at the polls. Unexpectedly high number of voters in states with early voting such as Florida have already encountered long waits.
Hilary Shelton, director of the National Association for the Advancement of Coloured People’s (NAACP) Washington bureau, said there could be a repeat of problems witnessed in some black inner cities in 2004, where voters waited for up to eight hours to cast their ballots.
In response to the expected high turnout among racial and ethnic minority voters, intense interest in the election and online rumours about unrest, the NAACP has written to election officials in every state asking them to try to prevent any problems that could lead to voters being "stymied" or "disenfranchised" such as too few voting machines or staff.
He was also concerned about the possibility of extra police presence causing intimidation.
"Our antennas go up in terms of what happens when law enforcement moves to provide additional security and support and what happens on election day and how that comes across.
"The issue we’re raising now is are they being sensitive to the issues and the possibilities of intimidation and disenfranchisement, which could very well come out of them being too heavy handed. (Sometimes) the wrong strategy by law enforcement can actually create a problem rather than prevent one and it is our hope that we don’t see those kinds of problems on election day and people are able to enjoy the security of our democracy."
Right-wing websites and blogs have been fuelling speculation about election unrest with unconfirmed reports of an online petition that pledges "civil disobedience" if Mr McCain wins.
Meanwhile, in a blog posting entitled ‘A McCain "Win" Will Be Theft, Resistance Is Planned’, David Swanson, Washington director of Democrats.com and a board member of Progressive Democrats of America, writes: "If your television declares John McCain the president elect on the evening of November 4th, your television will be lying.
"You should immediately pick up your pre-packed bags and head straight to the White House in Washington, DC, which we will surround and shut down until this attempt at a third illegitimate presidency is reversed.
"We may be there for days or weeks or months. But we must be there. We must be there by the millions. We must show each other, and the nation, and the world that we have had enough, that we will not stand for one more stolen election, that we will not give in to fear, lies, theft, and intimidation."
Mr Carville told The Hill that "a lot of Democrats would have a great deal of angst and anger," if Mr Obama lost. He predicted that on November 4, "the voting system all around the country is going to be very stressed because there’s going to be enormous turnout."