The Paris-based organization, Reporters Without Borders, says the tiny kingdom of Nepal accounted for half the number of cases of censorship in the world in 2005.
In a statement issued Monday, the organization says 425 journalists were arrested, attacked or threatened in the past year as Nepalese security forces "stopped at nothing" to monitor and silence journalists.
The statement says the pattern is continuing and at least six journalists have been detained, attacked, or threatened since the start of this year.
Vincent Brossell, Asia-Pacific regional head at Reporters Without Borders says Nepalese media are confronting every kind of infringement since King Gyanendra seized power nearly a year ago and tried to curb dissent.
"They use very traditional pressures like advertisement boycotts, threatening to withdraw the license, seizure of equipment, all sorts of tricks, warrants, and defamation cases, press laws have been changed, in a very drastic way and you can put journalists in detention for just defamation," said Mr. Brossell.
King Gyanendra took control of the government in February in a move he said was necessary to fight a Maoist revolt that aims to topple the monarchy.
The King's administration jailed many journalists, but eased some of the strict media curbs after lifting the state of emergency in April.
But in October, the government again cracked down on the media by imposing tough new laws banning the criticism of the King and imposing stiffer penalties for defamation.
The President of the Federation of Nepalese Journalists, Bishnu Nishturi, says journalists are facing increasing intimidation before local elections next month. Political parties are boycotting the polls and the Maoist rebels have vowed to disrupt them.
Mr. Nishturi says the police, army and local officials summon journalists, question them, and put pressure on them not to carry news about the poll boycott. He says journalists in outlying districts face greater harassment than those in Kathmandu.
Reporters Without Borders is urging the international community to stress the importance of respecting press freedom to the Nepalese government.
Mr. Brossell says the country's fiercely independent media has failed to toe the King's line, and needs support.
"Now the army is more involved in threatening or harassing journalists," he said. "Now the civil authorities are in charge of trying to put pressure on the editors and newsrooms . But the journalists in Nepal have been very courageous."
The Nepalese government has defended media curbs by saying they are intended to make the press more responsible.