Voters and civil rights groups reported long lines, isolated cases of malfunctioning equipment and some harassment at polling places in Tuesday's US presidential election, but fears of widespread violence at the polls did not materialize.

Nationwide, civil rights groups logged unusual levels of voter intimidation complaints, receiving about 20,000 calls through a national voter complaint hotline as of Tuesday afternoon. Democratic Party officials, however, said they did not see systemic voter suppression efforts.

Civil rights groups, who have enlisted 7,000 volunteers, said 40 percent of the calls they had received through a telephone hotline by early afternoon were from African-American and Latino voters.

Complaints over voter intimidation were especially prevalent in Florida, Georgia, Ohio, and Arizona, with a disproportionate share of those coming from minority voters.

"We are hearing more complaints about voter intimidation than we have in presidential cycles from prior years," said Kristen Clarke, president of the Lawyers' Committee for Civil Rights Under Law.

Republican presidential candidate Donald Trump has repeatedly said the election would be "rigged" and called on his supporters to watch for signs of fraud in urban areas, raising fears they could clash with minority voters. Numerous studies have found that voter fraud is exceedingly rare in the United States.

Some voters in Florida reported a heavy, unexplained police presence at several polling sites around the state. A group of students in the state were told their votes would not count because their college dormitory was considered a hotel.

In Ohio's Franklin County, the hotline received complaints that Somali American voters were told they would have to vote provisionally because their addresses did not match their identification cards - and then were told that the polling site had run out of provisional ballots.

Malfunctioning machines dominated voter complaints out of Pennsylvania, Arizona, New York, North Carolina, and Virginia.

Trump cited reports that polling machines were switching votes for Republican candidates to votes for Democrats as cause for concern that the election outcome might not be valid, once again warning of a rigged system.

Speaking to Fox News, Trump provided no evidence to support the allegations or say where polling machines were breaking down, though there were some problems in Pennsylvania.

Voters in at least four Pennsylvania counties said touch-screen voting machines were mistakenly switching votes. State officials, however, said they did not believe any ballots had been wrongly counted as a result of the machine malfunctions.

Absent on Tuesday were reports of the kind of hostile political displays, vandalism and violence that have cropped up regularly throughout the campaign. A polling station was locked down in Southern California after one person was killed and three wounded by gunfire, but police saw no immediate indication the incident was linked to Election Day.

A Nevada judge on Tuesday rejected a request from Trump for records from a Las Vegas polling place that his campaign said had improperly remained open last week to accommodate people who were lined up to vote. The judge agreed with a county attorney who argued that election officials already preserve records.

Tens of millions of voters have cast ballots to conclude an unusually bitter presidential campaign.

The federal government reduced its election monitoring program in the wake of a 2013 US Supreme Court decision that weakened federal oversight of states with a history of racial discrimination. Revised voting laws and lengthy court battles in many states also have left voters uncertain about when and where they can cast their ballot and whether they will need to present photo identification.

Pennsylvania is one of a handful of states that relies on electronic voting machines without a paper backup that would allow officials to double-check the outcome if it is challenged.

The machines can record votes incorrectly if they are not calibrated properly, a problem that is magnified as the screens degrade with age.