The U.S. presidential campaign has heated up over the racial unrest simmering in the country, with presumptive Democratic nominee Joe Biden accusing President Donald Trump on Tuesday of being "part of the problem" rather than working toward a solution.

"This president today is part of the problem and accelerates it," former Vice President Biden said in Philadelphia, a day after Trump threatened to deploy the military if necessary to crack down on violence that has accompanied a week of protests over the death of a black man in police custody in Minnesota.

While authorities are tightening security to prevent disorder, protest activities have shown no signs of waning, posing a fresh challenge to an administration that is already struggling to contain the coronavirus pandemic as the presidential election looms in November.

Former U.S. Vice President and 2020 Democratic presidential candidate Joe Biden smiles during a Super Tuesday primary night rally in Los Angeles on March 3, 2020. (Kyodo) 

On Tuesday, a large number of peaceful demonstrators were seen gathering near the White House and marching in New York and elsewhere, demanding an end to racism and unjust police practices following the inhumane treatment of George Floyd by a white police officer on May 25.

In video footage that quickly went viral, Floyd, 46, was seen gasping for air while the officer knelt on his neck for nearly nine minutes after his arrest for allegedly using a counterfeit bill. He was pronounced dead shortly afterward.

In a televised speech on Tuesday, Biden, who served under Barack Obama, the first African-American president in U.S. history, vowed to deal with the "systematic racism" and "growing economic inequity" that he said exist in the United States.

In seeking to express empathy for the grievances gripping the country, he contrasted himself with Trump, who has presented himself as a strongman during the ongoing crisis.

U.S. President Donald Trump walks out of the White House to make a statement to the press about restoring "law and order" on June 1, 2020 in Washington, DC. (PHOTO NOT FOR SALE)(Getty/Kyodo)

Trump has tweeted remarks such as "looting leads to shooting," a message widely interpreted as a threat against the protestors. Similar remarks tweeted by Trump were quickly flagged by Twitter Inc. as violating its rules over "glorifying violence."

Trump's plan to deploy military forces against American citizens if necessary has also stirred controversy, with critics saying it may simply inflame the situation rather than calm it.

"I know there's enormous fear and uncertainty and anger in the country. I understand," Biden said, adding, "I know what it means to have that black hole in your chest, where your grief is being sucked into it."

Biden, who endured the personal tragedies of losing two of his four children -- one who died along with his first wife in a car accident, and another from brain cancer -- has often been seen as a politician capable of communicating compassion.

While acknowledging that "nobody will get it right" all the time in a job like the presidency, Biden said, "I won't traffic in fear and division. I won't fan the flames of hate. I'll seek to heal the racial wounds that have long plagued our country, not use them for political gain."

Biden went to Philadelphia, Pennsylvania, in his first trip in months out of Delaware, where he has stayed as the novel coronavirus rages in the country.

Trump defended his approach, tweeting on Tuesday that the U.S. capital, which saw some areas near the White House torched on Sunday, had "no problems last night."

"Many arrests. Great job done by all. Overwhelming force," he said, adding a parenthetical "thank you President Trump!"

The president also hit back at Biden, saying, "Sleepy Joe has been in politics for 40 years, and did nothing. Now he pretends to have the answers."

"Weakness will never beat anarchists, looters or thugs, and Joe has been politically weak all of his life. LAW & ORDER!" Trump tweeted.

So far, National Guard troops, many of whom are civilian employees and respond to domestic emergencies and other situations, have been mobilized in states across the country to support police forces confronting violence at the protests.