Polling stations have opened across dozens of American states in a presidential election that is set to be a close race between Democratic candidate Vice-President Kamala Harris and Republican Donald Trump.

By Tuesday evening, Kabul time, polling stations had opened in more than three dozen states across the eastern and central US - including Washington DC, Florida, Illinois, Georgia, Alabama, Delaware, Maryland, Massachusetts, Pennsylvania and Missouri.

Alongside the presidential race, hundreds of congressional seats are also at stake, which will shape the party balance in the House and Senate.

Democratic candidate Kamala Harris and Republican candidate Donald Trump are polling neck and neck in the seven swing states which will likely decide the presidential race.

Currently, experts say the race could go either way.

If Donald Trump wins this election, he will enter the White House for the second time.

If Kamala Harris wins, she will become the first female president in the history of the United States. She will also be the first Asian American president.

But as millions of Americans get ready to vote, the public remains on edge - not only about the vote but about what might follow.

For four years, Trump and his allies have prepared to challenge the outcome if he loses again.

They have spent months filing lawsuits, laying the groundwork to contest ballots, results and the eligibility of voters. They have recruited thousands of volunteers to monitor polling places, drop boxes and counting facilities.

And, without evidence, they have claimed that the cheating has already begun — priming their staunchest supporters for confrontation, intimidation and, in the worst case, violence.

However, state and federal authorities are prepared. They have changed laws to make it harder to challenge certified results, strengthened security at election facilities and launched massive message campaigns to encourage public trust in US elections.

As a tense America votes on Tuesday for either Republican Donald Trump or Democratic Vice President Kamala Harris for president, concerns about potential political violence have prompted officials to take a variety of measures to bolster security during and after Election Day.

Many of the most visible moves can be seen in the battleground states that will decide the presidential election, states like Nevada where protests by Trump supporters broke out after the 2020 election.

This year, a security fence rings the scene of some of those protests - the Las Vegas tabulation center.

A defense official said on Monday that Alabama, Arizona, Delaware, Iowa, Illinois, North Carolina, New Mexico, Oregon, Wisconsin and Washington state have current National Guard missions while Washington DC, Colorado, Florida, Hawaii, Nevada, Oregon, Pennsylvania, Tennessee, Texas and West Virginia have troops on standby.

In Arizona, a similar metal fence has been erected at Maricopa County vote tabulation center in downtown Phoenix, a flashpoint in 2020 for rigged election conspiracy theories and threats against election officials.

County Sheriff Russ Skinner said his department will be on "high alert" for threats and violence and he has instructed staff to be available for duty.

"We will have a lot of resources out there, a lot of staff, a lot of equipment," he added, noting deputies will use drones to monitor activity around polling places and snipers and other reinforcements will be on standby for deployment if violence appears likely.

He said "polarization" becomes more intense in the days after the election so law enforcement will remain on heightened alert and "there will be zero tolerance on anything related to criminal activity".

Concerned about the potential for protests or even violence, several Arizona schools and churches that served as voting centers in the past will not serve as polling stations this year, a local election official told Reuters.

Precautions stretch beyond the battleground states. Oregon and Washington state authorities have said they have activated the National Guard. Some storefront windows in Washington, DC and elsewhere have been covered by plywood.

Back in Las Vegas, Faviola Garibay surveyed the security fence around the linen-colored building where Clark County officials tabulate the votes and where voters such as her can drop election ballots.

"The fencing, the presence of police here, it seems secure," she said. "I feel safe voting."

Israeli airstrikes killed at least 31 people in the Gaza Strip on Sunday, Palestinian medics said, with nearly half of the deaths in northern areas where the army has waged a month-long campaign it says is aimed at preventing Hamas from regrouping, Reuters reported.

Palestinians said the new aerial and ground offensives and forced evacuations were "ethnic cleansing" aimed at emptying two northern Gaza towns and a refugee camp of their population in order to create buffer zones. Israel denies this, saying it is fighting Hamas militants who launch attacks from there.

Medics said at least 13 Palestinians were killed in separate attacks on houses in Beit Lahiya town and Jabalia, the largest of the enclave's eight historic camps and the focus of the army's new offensive.

The rest were killed in separate Israeli airstrikes in Gaza City and in southern areas, including one in Khan Younis, which health officials said had killed eight people, including four children.

Later on Sunday, health officials at the Kamal Adwan Hospital near Beit Lahiya said the facility came under Israeli tank fire and that one child hospitalized at the hospital was critically wounded.

Hussam Abu Safiya, the hospital's director said the incident took place after a delegation from the World Health Organization visited the facility and evacuated some patients.

He said while evacuating the wounded was important, it was more important to dispatch specialized medical teams to north Gaza hospitals that have become overwhelmed by the number of casualties.

Abu Safiya said the tank fire hit the water supplies, the courtyard, and the neonatal intensive care unit.

COGAT, the Israeli army's Palestinian civilian affairs agency, said the explosion resulted from an explosive device planted by Palestinian militants and not an Israeli attack.

"The terrorist organizations continue to exploit civilian infrastructure, medical facilities, and international aid organizations for their terror activities," a COGAT statement said late on Sunday.

Hamas has repeatedly denied using civilian facilities such as hospitals, schools, and mosques, for military purposes.

On Saturday, the Israeli military sent a new army division to Jabalia to join two other operating battalions, a statement said. It said hundreds of Palestinian militants have been killed in the "battles" since the raid began on Oct. 5, read the report.

Meanwhile, COGAT said it facilitated the launch of the second round of a polio vaccination campaign in northern Gaza on Saturday and that 58,604 children have received a dose.

The Gaza health ministry said Israel's military offensive in northern Gaza was stopping them from vaccinating thousands of children in Jabalia, Beit Lahiya, and Beit Hanoun.

It said one clinic had come under Israeli fire while parents brought their children for the anti-polio dose on Saturday and that four children had been injured.

The head of the World Health Organization said in a statement the clinic incident took place despite a humanitarian pause agreed upon by the two warring parties, Israel and Hamas, to allow the vaccination campaign.

"A @WHO team was at the site just before. This attack, during a humanitarian pause, jeopardises the sanctity of health protection for children and may deter parents from bringing their children for vaccination," Director-General Tedros Adhanom Ghebreyesus said in a post on X on Saturday.

"These vital humanitarian-area-specific pauses must be absolutely respected. Ceasefire!" he said.

The Israeli military, which had no immediate comment on Tedros' remarks, said it was checking the report about the clinic.

A larger ceasefire that would end the war and see the release of Israeli and foreign hostages held captive in Gaza as well as Palestinians jailed by Israel remains remote due to disagreements between Hamas and Israel.

Hamas wants an agreement to end the war permanently, refusing recent offers for temporary truces, while Israeli Prime Minister Benjamin Netanyahu says war can only end when Hamas is eradicated, Reuters reported.

The war erupted after Hamas-led militants attacked Israel on Oct. 7, 2023, killing some 1,200 people and taking 251 hostages back to Gaza, according to Israeli tallies.

Israel's retaliatory offensives have killed more than 43,300 Palestinians and reduced most of Gaza to rubble.