The four are being held on drugs or weapons charges.

According to Denver TV station CBS4, one of the suspects told authorities they were "going to shoot Obama from a high vantage point using a ... rifle sighted at 750 yards".

Law enforcement officials told CBS4 one of the suspects "was directly asked if they had come to Denver to kill Obama. He responded in the affirmative."

The apparent plot began emerging on Sunday morning when police just outside of Denver arrested 28-year-old Tharin Gartrell. He was driving a rented pickup truck in an erratic manner, according to sources.

CBS4 said police found two high-powered, scoped rifles in the car along with camouflage clothing, walkie-talkies, a bulletproof vest, a spotting scope, licences in the names of other people and methamphetamine. One of the rifles is listed as stolen from Kansas.

Subsequently authorities went to a local hotel to contact an associate of Gartrell's. But that man, who was wanted on numerous warrants, jumped out of a sixth floor hotel window.

Law enforcement sources say the man broke an ankle in the fall and was captured moments later.

Sources say he was wearing a ring with a swastika, and is thought to have ties to white supremacist organisations.

CBS4 said a third man - an associate of Gartrell - and the hotel jumper was also arrested. He told authorities that the two men "planned to kill Barack Obama at his acceptance speech".

A woman is also under arrest.

Multiple law enforcement agencies are investigating the plot, including the Secret Service, FBI and the joint terrorism task force.

The Australian