Saudi intellectuals on Wednesday urged the kingdom's crown prince to order the release of a liberal writer accused of insulting Islam via his Twitter account.
A petition, signed by around 500 people and addressed to Salman bin Abdul Aziz, called for Turki al-Hamad's "immediate and unconditional release."
"We hope for, demand and expect a quick decision to be made to correct this grave error that has been committed against" Hamad, it said.
It slammed his arrest as "unjust... condemnable, reprehensible, shameful, and unacceptable."
Hamad was arrested -- on the orders of Interior Minister Prince Mohammed bin Nayef bin Abdel Aziz, who was tipped off by a religious organisation -- for alleged insults to Islam he made on Twitter, his family said on December 24.
The comments he posted had attacked radical Islamists he said were twisting the Prophet Mohammed's "message of love," and what he described as "a neo-Nazism which is on the rise in the Arab world -- Islamic extremism".
The postings provoked fierce debate on social networking sites in Saudi Arabia between his supporters and detractors.
The petition's signatories included Manal al-Sherif and Najla Hariri, two female activists who defied the kingdom's driving ban on women, and liberal columnist Ahmad Adnan.
It called for "a public apology" to Hamad, saying he was targeted by online "incitement" campaigns to try him.
The petition said the interpretation of his comments and subsequent arrest were a "flagrant violation of the Charter of Human Rights and in an explicit assault on an individual’s right to free speech."
Online activist Raif Badawi, another Saudi, was arrested in June in Jeddah and accused of apostasy, which carries the death penalty in the Gulf kingdom.
Badawi helped set up a liberal Saudi website, which declared a "day of liberalism" on May 7, calling for protests against the stranglehold of religious officials on public life in the strict Sunni-ruled monarchy.
Amnesty International had condemned Badawi's prosecution.