Mikhlif al-Shammari, 58, was sentenced to five years in prison at Saudi Arabia's Specialized Criminal Court, on June 17.

"Al-Shammari is the latest in a lengthening line of Saudi human rights activists hauled before the courts and branded as criminals for exercising their right to free speech," Joe Stork, the HRW's deputy Middle East director said.

Stork added that the Saudi King Abdullah needed to reform the criminal justice system to end the abuses, “unless he wants his legacy to be repression rather than reform."

According to the New York-based watchdog, Shammari, arrested in 2010, was convicted of “sowing discord” and other offences. He was also barred from traveling for 10 years.

The HRW said that the activist was sentenced "based on his writings and exposure of human rights abuses."

He was reportedly arrested and convicted based on "articles he wrote and published in 2009 and 2010, criticizing corruption, double standards, and the hypocrisy of some religious figures."

Shammari had been known for campaigning to improve ties with the kingdom’s Shia minority.

More than 40,000 political prisoners, mostly prisoners of conscience, are reportedly in jails across Saudi Arabia.

Meanwhile, human rights activists have said that more than 120 prisoners in Saudi Arabia had gone on hunger strike to express their anger at inhumane prison conditions.

The strike is expected to continue for at least five weeks, according to human rights activists.

In Saudi Arabia, protests and political gatherings of any kind are prohibited.

Since February 2011, protesters have held demonstrations on an almost regular basis in Saudi Arabia, mainly in Qatif and Awamiyah, primarily calling for the release of all political prisoners, freedom of expression and assembly, and an end to widespread discrimination.