Denver, Colorado - A federal attorney will discuss later Tuesday details of the arrest of three men in connection with what authorities believe could have been a threat against presidential candidate Barack Obama, officials said.

In a statement earlier Tuesday, US Attorney Troy Eid said federal police were working "hand in glove" with local police in Aurora, a town on the outskirts of Denver where the initial arrest of a man with firearms and drugs took place.

But Eid sought to allay fears that there was a serious threat.

"We can say this: We're absolutely confident there is no credible threat to the candidate, the Democratic National Convention, or the people of Colorado," Eid said.

The Democratic Party's presidential nominating convention opened Monday evening in Denver, Colorado. On Thursday, US Senator Barack Obama is scheduled to deliver an acceptance speech at a football stadium to a crowd of 76,000, becoming the first African-American to win a major-party presidential nomination.

The case began when local police stopped a suspected drunk driver in a pickup truck in Aurora. The 28-year-old man was found in possession of methamphetamine, the Denver Post reported.

Police also seized two rifles, a high-powered scope-attachment and ammunition from the vehicle.

He led them to his hotel room in Glendale, Colorado, where as police arrived a second man leapt from a sixth-storey window to an awning four floors lower, falling to the ground and suffering a leg injury.

The second suspect and a third man arrested at a Denver hotel were both held on outstanding warrants.

Federal, state and local police are a strong, visible presence throughout Denver, with police dressed in riot gear on many street corners or hanging on to pickup trucks that rush to possible threat scenes.

Police and rescue helicopters have been periodically running practice missions over Denver in preparation for the convention, and local residents say some of the aircraft have police perched in the open doors with guns at the ready.

The US Secret Service, which protects the US president, other top government officials and presidential candidates, was investigating, along with other federal police agencies.

"Behavior like this can be characterized as highly suspicious, the ammo, the weapons, clearly would give us great concern," Aurora Police spokesman Marcus Dudley said late Monday.

In an incident separate from the alleged plot against Obama, Denver police early Tuesday arrested 100 rock-bearing demonstrators in Civic Center Park, which has been set aside for protests. They were charged with obstruction of streets and disobedience of "lawful order."