The FBI is investigating a possible assassination plot against the Democratic candidate Barack Obama which was reportedly thwarted by the local police.

The attempt was supposedly timed to occur during Mr Obama's acceptance speech before an audience of some 70,000 at Invesco Field on Thursday night. Law enforcement officers told a local television station that one of the suspects "was directly asked if they had come to Denver to kill Obama. He responded in the affirmative."

Police stopped a swerving vehicle early on Sunday morning and inside reportedly discovered drugs, ammunition, a bulletproof vest, two rifles, walkie-talkies and a rifle scope. Three men were subsequently arrested.

The authorities said there were "federal implications" in their actions but they were reluctant to confirm that an assassination plot had been halted in its tracks.

Those arrested were Tharin Gartrell, the 28-year-old driver; 32-year-old Nathan Johnson who was arrested in a hotel some three hours later; and Shawn Adolph, a 33-year-old who was hospitalised after jumping out a hotel window as the police sought him. “We're absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado,” said Troy Eid, the US chief prosecutor for Colorado.

The FBI is also investigating threats made against Barack Obama, officials said.

"It's premature to say that it was a valid threat or that these folks have the ability to carry it out," said a US government official.