Senator Elizabeth Warren in Lawrence, Massachusetts.Photo / AP US Senator Elizabeth Warren formally launched her presidential bid by re-emphasising her signature issues of fighting economic inequality and corporate wrongdoing, in an effort to move beyond new questions about her previous claims that she was a Native American.

The outdoor rally of about 3500 in Lawrence, Massachusetts, was intended to position Warren as the leader of a renewed crusade against what she called the "corrupt" influence of large companies and powerful politicians.

"The story of Lawrence is a story about how real change happens in America," Warren said. "It is a story about power — our power — when we fight together."

The announcement came after a difficult week for Warren. The Washington Post published a document that shows, for the first time, Warren's handwritten assertion that she was an "American Indian" on a 1986 registration card she filled out for the Texas bar. Warren in recent days offered apologies for claiming Native American identity, first privately to the leader of the Cherokee Nation and then publicly.

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Warren sought to carve out a distinctive position in the crowded Democratic field, citing achievements like the creation of the Consumer Financial Protection Bureau, which regulates banks and other financial institutions.

"It won't be enough to just undo the terrible acts of this administration," Warren said. "We can't afford to just tinker around the edges — a tax credit here, a regulation there. Our fight is for big, structural change."

Some of those who attended the kickoff had concerns about the strength of her candidacy, given the identity issue and President Donald Trump's aggressive campaign style. Trump campaign manager Brad Parscale said Warren has been "exposed as a fraud by the Native Americans she impersonated and disrespected to advance her professional career".

Trump tweeted: "Today Elizabeth Warren, sometimes referred to by me as Pocahontas, joined the race for President. Will she run as our first Native American presidential candidate, or has she decided that after 32 years, this is not playing so well anymore? See you on the campaign TRAIL, Liz!"

"Trump is an intimidating guy to take on," said Hugh Stinson, 40. "Warren has left herself open on a couple fronts to pile on." "I want a candidate who will win. It's a challenging time," said Pamela Baldwin, 55, who said she likes Warren as a senator but believes sexism will prevent a woman from becoming president in 2020.

- additional reporting AP