US Senator Bernie Sanders, an independent from Vermont, kicked off his long-shot bid for the 2016 Democratic presidential nomination on Tuesday, vowing to address the "immoral" income and wealth disparity in the US.

In a bold challenge to leading Democratic candidate Hillary Rodham Clinton, the 73-year-old Sanders called on liberals in the Democratic Party and others who want to change America's "rigged economy" that favours the rich to throw their support to him.

The two-time senator vowed to rebuild the middle class and reduce the influence of wealthy Americans and businesses on US elections.

Speaking to about 5,000 supporters in Burlington, Vermont – where he served as mayor in the 1980s – Sanders called his presidential bid a "political revolution to transform our country economically, politically, socially and environmentally."

"Today, here in our small state – a state that has led the nation in so many ways – I am proud to announce my candidacy for president of the United States of America," he declared.

He told the crowd that "this country faces more serious problems today than at any time since the Great Depression and, if you include the planetary crisis of climate change, it may well be that the challenges we face now are direr than any time in our modern history."

After Vermont, he said his next stops will be in New Hampshire, Iowa and Minnesota.

"Let's be clear. This campaign is not about Bernie Sanders. It is not about Hillary Clinton. It is not about Jeb Bush or anyone else. This campaign is about the needs of the American people, and the ideas and proposals that effectively address those needs," he said.

He outlined the issues he will be focusing on during the campaign including economy, jobs and climate change.

The US is the wealthiest nation in the world but only a handful control most of the wealth, he said.

"In America we now have more income and wealth inequality than any other major country on Earth, and the gap between the very rich and everyone is wider than at any time since the 1920s," he said, adding that the top one-tenth of 1 percent owns "almost as much wealth as the bottom 90 percent, and when 99 percent of all new income goes to the top 1 percent."

He said this income and wealth disparity is "immoral."

"This type of rigged economy is not what America is supposed to be about. This has got to change and, as your president, together we will change it," he said.

Sanders bewailed the unemployment rate in the US, saying that in reality, it's close to 11 percent.

"Today, shamefully, we have 45 million people living in poverty, many of whom are working at low-wage jobs. These are the people who struggle every day to find the money to feed their kids, to pay their electric bills and to put gas in the car to get to work," he said.

He also criticized how the rich can "now own the US government" through political funding after the Supreme Court decision on Citizens United.

"American democracy is not about billionaires being able to buy candidates and elections. It is not about the Koch brothers, Sheldon Adelson and other incredibly wealthy individuals spending billions of dollars to elect candidates who will make the rich richer and everyone else poorer," he said.

He also vowed to raise the minimum wage to $15 an hour from the current $7.25 an hour.