SAN FRANCISCO (AFP) - The United States authorities said on Wednesday they had shut down the Silk Road website and arrested its owner on charges he ran an online black market for illegal drugs and services.

Ross William Ulbricht, also known as "Dread Pirate Roberts," was arrested on Tuesday in San Francisco after the website was shuttered, the Justice Department said in a statement.

Prosecutors said they seized approximately US$3.6 million (S$4.5 million) worth of Bitcoins in the largest ever seizure of the virtual currency.

"The Silk Road website has served as a sprawling black market bazaar where illegal drugs and other illicit goods and services have been regularly bought and sold by the site's users," Federal Bureau of Investigation special agent Christopher Tarbell said in a criminal complaint filed in a federal court.

From about January 2011, Ulbricht ran a marketplace that hawked heroin, cocaine, LSD, and methamphetamine as well as hacker tools such as software for stealing passwords or logging keystrokes on people's machines, according to the court documents.

Prosecutors also charged that in March of this year, Ulbricht tried to hire someone to kill a Silk Road user who threatened to expose the identities of others using the website.