A Bahraini watchdog says the country's leading activist, Nabeel Rajab, has been charged with publishing "false news and statements and malicious rumors that undermine the prestige of the kingdom," a day after his letter appeared in The New York Times.

The Bahrain Institute for Rights and Democracy reported the charges on Monday.

Rajab's "Letter from a Bahraini Jail" was published on Sunday in which he accuses Bahrain of subjecting "its people to imprisonment, torture and even death for daring to desire democracy."

Rajab has been detained since mid-June over his criticism of Saudi Arabia's war in Yemen and the condition of detainees at Bahrain's Jaw prison.

The state-run Bahrain News Agency reported the charges but said "the accused" denied any relation with what was published under his name during interrogation.