Mr Biden has 243 Electoral College votes, giving him the edge in the race to accumulate the 270 needed to win the White House. Mr Trump has 214.
In the United States election, voters decide state-level contests rather than a single, national one. Each US state gets a certain number of electoral college votes partly based on its population, with a total of 538 up for grabs.
If Mr Trump does lose Wisconsin (10 electoral college votes), he must win Georgia (16 votes), North Carolina (15), Pennsylvania (20) and either Arizona (11) or Nevada (6) to prevail.
Officials in Georgia said they would keep going all night until counting was finished, but as of midnight local time (05:00 GMT) they reported that there were still about 90,000 votes to count. At that time President Trump was leading by some 31,000 votes.
In Arizona, Mr Biden was leading by about 80,000 votes, with further results expected on Thursday . CBS has categorised it as a “likely” win for the Democrat. Supporters of Mr Trump gathered outside a vote counting centre in Maricopa County (which includes Phoenix, the largest city in the state), where officials vowed to “continue our job” .
An update on the count in Nevada – where the candidates were neck-and-neck – is not expected until Thursday at 17:00 GMT (09:00 local time), while in Philadelphia, counting is not expected to finish for several days.
What about the legal challenges?
The Trump campaign said the president would formally request a Wisconsin recount, citing “irregularities in several Wisconsin counties”.
Incomplete results indicate the margin between Mr Trump and Mr Biden in Wisconsin is less than one percentage point, which allows a candidate to seek a recount.
The campaign also filed a lawsuit in Michigan to stop counting there because it contended it had been denied “meaningful access” to observe the opening of ballots and the tally.
In Detroit, Michigan, police were called on Wednesday afternoon to guard the doors to a vote-counting facility as some protesters outside demanded access to monitor the process. According to the Detroit Free Press, there were already some 200 people observing the vote inside the building.
Officials were seen covering up the windows to the TCF Center, where postal ballots were being tabulated.
Could the election be decided in courts?
The Trump campaign also filed two lawsuits in Pennsylvania to halt all vote counting “until there is meaningful transparency”.
The president has a three-point lead in the Keystone State, but many thousands of votes remain to be counted.
Mr Trump is also suing Georgia to halt the vote count there. His campaign said a Republican poll observer in the southern state had witnessed 53 late absentee ballots being illegally added to a pile of votes in Chatham County.
In 2016, Mr Trump’s wins in Wisconsin, Michigan and Pennsylvania sealed his victory and took him to the White House.
In the early hours of Wednesday, the president claimed he had won the election despite many uncounted votes remaining. The speech brought strong criticism from both Democrats and Republicans.
Mr Trump also said he was prepared to take the election to the Supreme Court, and his campaign is asking Republican donors to help fund legal challenges. Such challenges start at a state level but could eventually lead to the top court in the US.
Republican National Committee Chairwoman Ronna McDaniel said: “The fight’s not over. We’re in it.”
Mr Biden’s running mate, Kamala Harris, tweeted asking supporters to contribute $5 to help pay for litigation that could “stretch on for weeks”.
Biden campaign senior legal adviser Bob Bauer said there were no grounds for Mr Trump to invalidate lawful ballots.
What were the other key results?
Mr Biden’s hopes of a decisive early victory on election night were thwarted as Mr Trump defied pollsters’ predictions by over-performing in key battlegrounds.
The president held several important states, including Texas, Ohio and Iowa.
He also comfortably won his adopted home state of Florida, the most hotly fought battleground of the night, despite two visits there by Mr Biden’s ally and former boss, ex-President Barack Obama.
But Mr Biden fended off his rival’s attempts to pick up New Hampshire and Minnesota.
What did the exit polls reveal?
Some political analysts were surprised by data that showed Mr Trump doubled his support among black voters to 12%, compared with four years ago.
He also boosted his vote share among Hispanic men to 36%, according to the exit polls, compared with 28% in 2016.
African-Americans and Latinos are two vital electoral blocs for Democrats.
The president shed votes, though, among white men, the demographic that propelled him to the White House four years ago.
However, Mr Trump also increased his support significantly among white women (55% on Tuesday versus 47% in 2016).
What about the congressional races?
Democrats dreamed of an electoral blue wave sweeping away a Republican party that has been recast in Mr Trump’s image. But those hopes were dashed.
Their chances of winning Senate control appeared to be dwindling as the dust settled from election day. Democrats won two seats in Colorado and Arizona, but lost another in Alabama.
A Senate race in North Carolina remained undecided, and another in Georgia was heading for a run-off vote in January.
Senate Majority Leader Mitch McConnell said he felt “pretty good” about the remaining contests. Republicans currently control the chamber by 53-47.
Mr McConnell and Trump ally Lindsey Graham were both re-elected in their seats of Kentucky and South Carolina respectively.
Republicans also held on to other Senate seats in Maine, Montana, Texas and Iowa.
There was another unexpected setback for Democrats in House of Representatives races. The party had hoped to gain 15 seats in the lower chamber of Congress, which they currently control.
Instead, they face a reduced majority after seven of their incumbents lost and the party failed to beat a single Republican.
One of the incoming Republican lawmakers, in Georgia, has been described as a supporter of the fringe, pro-Trump QAnon conspiracy theory.
– BBC
