Saudi authorities have stepped up their crackdown on online dissidents, Human Rights Watch said, alleging that prosecutors and judges use "vague law" to charge citizens for peaceful tweets and social media comments.

The New York-based rights organisation on Sunday called on the government to end the crackdown and live up to its obligations to respect free speech.

Three prominent lawyers were convicted of criticising the Justice Ministry last month and sentenced to prison terms of between five and eight years.

Instead of pursuing their peaceful online critics, Saudi officials would be better employed in carrying out much-needed reforms Sarah Leah Whitson, Human Rights Watch

Police also detained a liberal women’s rights activist in connection with tweets that allegedly criticised religious officials and promoted the right of Saudi women to drive.

"These prosecutions show just how sensitive the Saudi authorities have become to the ability of ordinary citizens to voice opinions online that the government considers controversial or taboo," said Sarah Leah Whitson, Middle East and North Africa director at Human Rights Watch (HRW).

"Instead of pursuing their peaceful online critics, Saudi officials would be better employed in carrying out much-needed reforms."

The organisation said that Saudi authorities are using vague provisions of a 2007 anti-cybercrime law to charge and try Saudi citizens.

Article 6 criminalises "producing something that harms public order, religious values, public morals, the sanctity of private life, or authoring, sending, or storing it via an information network," and imposes penalties of up to five years in prison and a fine of up to three million Saudi Riyals (US$800,000).

'Attacking Sharia'

According to court documents that HRW has reviewed, prosecutors based their entire case against the three lawyers – Abdulrahman al-Subaihi, Bander al-Nogaithan, and Abdulrahman al-Rumaih – on tweets that officials deemed critical of the Justice Ministry.

The indictment accused the lawyers, among other charges, of "attacking the Sharia judicial system and its independence, and undermining its authority by interfering in the [disciplinary] proceedings of Judge Mohammed Al Abdulkareem."

Human Rights Watch said none of the tweets cited as evidence against the three lawyers incited violence.

One of the lawyers said on Twitter: "The performance [of the Justice Ministry] is catastrophic, packaged in lies and media fraud unprecedented in the history of Saudi ministries."

Another had tweeted: "… the kingdom lives in 2013 but its judiciary lives in the darkness of the Middle Ages in its processes and management."

A third commented: "… $5 billion to Egypt while at the same time Saudi women clean bathrooms."

Saudi authorities have sanctioned other lawyers in 2014 for social media activity, including Waleed Abu al-Khair, who was sentenced to 15 years in prison in July solely on account of his peaceful criticism of Saudi human rights abuses in media interviews and on social media.

Suad al-Shammari, another activist, was arrested on October 28 while attending an investigation session at the Bureau of Investigation and Prosecution in Jeddah.

HRW said Shammari was facing the charge of "insulting the messenger and the hadith" in connection with the 2013 tweets that allegedly criticised religious authorities and called for Saudi women to be allowed to drive cars.