Rights activists across the United States held a series of protests on Thursday demanding the closure of Guantanamo Bay as a hunger-strike at the jail entered a third month.

In a day of action aimed at drawing attention to the plight of prisoners at the facility, demonstrators -- many wearing orange jumpsuits famously worn by Guantanamo inmates -- demanded US President Barack Obama close the jail.

The protests came as a group of 25 rights organizations wrote an open letter to the US leader.

The letter, whose signatories included Amnesty International, Human Rights Watch, and the American Civil Liberties Union, said the majority of 166 prisoners held at Guantanamo were now on hunger strike.

"The situation is the predictable result of continuing to hold prisoners indefinitely without charge for more than 11 years," it said.

"We urge you to begin working to transfer the remaining detained men to their home countries or other countries for resettlement, or to charge them in a court that comports with fair trial standards."

Nine protesters gathered outside the White House in Washington to demand Guantanamo's closure, while around 30 attended a New York demonstration. Similar protests took place in San Francisco, Los Angeles and Chicago.

Zeke Johnson, Director of Amnesty International USA's Security with Human Rights Campaign, said Guantanamo prisoners should be released or put on trial without delay.

"Death shouldn't be the only way out of Guantanamo," Johnson said. "The men must either be charged and fairly tried in federal court, or be released.

"It's time for President Obama to get serious about closing the detention facility."

The jail at Guantanamo was opened in 2002 to house prisoners rounded up in the "War on Terror" of President George W. Bush's administration following the September 11 attacks on New York and Washington the previous year.