Clinton, 57, had just started her address on social security to the Amherst Chamber of Commerce when she fainted, her office said in a statement.

The junior senator from New York had informed the audience she was feeling unwell after contracting a stomach virus.

"She felt weak, needed to sit down, and then fainted briefly," the statement said. "She received immediate medical attention at the site."

Sticking to her original schedule, Clinton then moved on to a Catholic college in Buffalo where she delivered a speech on health issues.

The senator is widely seen as a leading candidate for the Democratic presidential nomination in 2008.

Her husband, former US president Bill Clinton, underwent a quadruple heart bypass in September.