Former US Vice President Joe Biden has condemned "anger" within his own Democratic Party and pledged to work to unify the country in the wake of incumbent Donald Trump's presidency.

At a rally in downtown Philadelphia to launch his 2020 presidential bid, Mr Biden made US President DonaldTrump his central target, blasting him as "the divider-in-chief."

But he also chided other Democratic presidential candidates in the field, suggesting that anger toward Mr Trump within his party was not enough to win the presidential election next year.

His message, Mr Biden said, was expressly aimed at Democratic, Republican and independent voters alike.

"Some of the really smart folks say Democrats don’t want to hear about unity," he said. "They say Democrats are so angry, and that the angrier your campaign will be, the better chance you have to win the Democratic nomination. Well, I don’t believe it."

About 6,000 people attended the rally, which had the feel of a general election event. With his poll numbers currently swamping the rest of the Democratic field, Mr Biden has often acted as if his current opponent is Mr Trump and not the other 23 Democrats vying for the party's nomination.

"If the American people want a president to add to our division, to lead with a clenched fist, closed hand, a hard heart, to demonise the opponents and spew hatred - they don’t need me, they've got President Donald Trump," Mr Biden told the crowd.

Democratic nominating contests begin next February, giving the dynamics of the race plenty of time to shift.

However Mr Biden, 76, has opened up a more than 20-point lead over his nearest rival, Senator Bernie Sanders, in several public opinion polls.

Mr Biden, a US senator for 30 years and a two-term vice president under Barack Obama, has argued he is best positioned to take on Mr Trump next year.

Mr Biden also answered critics who have mocked his pledge to work with Republicans as unrealistic should he win the White House.

"I'm going to say something outrageous," he said. "I know how to make government work."

Mr Biden has established his campaign headquarters in Philadelphia, illustrating the importance of Pennsylvania to Democratic hopes next year.

Mr Trump, who narrowly won the state over Hillary Clinton in 2016, will hold an event of his own on Monday in northeast Pennsylvania.

Mr Biden will not have the luxury of shrugging off the rest of the Democratic field much longer.

In recent weeks, he has been criticized by Senator Kamala Harris for his past support for the 1994 crime bill that critics say led to mass incarceration of African-Americans, by Mr Sanders for his support of the North American Free Trade Agreement, and by Senator Elizabeth Warren for his ties to the credit-card industry.

With Mr Biden the clear front-runner, those attacks are likely to intensify.

However he said he would keep his focus on Mr Trump and not his rivals for the nomination.

"You will not hear me speak ill of another Democrat," he said.