WASHINGTON (AFP) - US Senator Bernie Sanders launched his populist, liberal campaign for the White House on Thursday, savaging an American economic system that creates "immoral" and unsustainable disparities between rich and poor.

"We're in this race to win," the 73-year-old Sanders, an independent lawmaker who is little known outside political circles and his home state of Vermont, told reporters at a low-key, rushed announcement on the lawn outside the US Capitol, where he has served in Congress since 1991.

By jumping into the presidential race, the self-described socialist Sanders presents a long-shot challenge to Democratic frontrunner Hillary Clinton but said he relishes the chance to go toe-to-toe with her in "serious debates over serious issues - not political gossip."

For years Sanders has warned of the growing economic gap in America, stressing that "millionaires and billionaires" run a rigged system that benefits the wealthy at the expense of huge numbers of working-class and poor citizens.

"Ninety-nine per cent of all new income generated in this country is going to the top one percent," he said.

"That type of economics is not only immoral, it's not only wrong, it is unsustainable."

Sanders is vehemently opposed to recently implemented laws that relaxed campaign finance rules, which he said allows billionaire donors to buy candidates of their choosing.

And he reiterated his fierce opposition to a massive trade pact under negotiation with Asia-Pacific nations, saying the deal would cost American jobs.

Sanders, whose father was a Jewish immigrant from Poland, was an eight-year mayor of Burlington, Vermont in the 1980s following a string of election losses for Congress and governor.

He eventually won a seat in Congress in 1990. He voted against authorising the use of military force against Iraq in 1991 and again in 2002, when Clinton voted in support of the Iraq war resolution.