"The arrest of Julian Assange is meant to send a message to all Americans and journalists: be quiet, behave, toe the line. Or you will pay the price," Gabbard said in a tweet on Thursday.
Earlier, UK authorities on behalf of the United States arrested Assange inside the Ecuadorian Embassy in London while US authorities unsealed an indictment charging the WikiLeaks founder with computer intrusion conspiracy, which carries a maximum sentence of five years in prison.
Assange gained fame after WikiLeaks published a large number of leaked files, including some referring to US military operations in Afghanistan and Iraq, and conditions in the Guantanamo detention camp.
Since 2012, Assange had been residing in the Ecuadorian Embassy in London to avoid extradition to Sweden, where he faced a probe over allegations of sexual offenses.
Assange denied those allegations and called them politically motivated. While the Swedish police dropped the investigation in 2017, Assange was still wary of being extradited to the United States.