Mohammad al-Qahtani, one of 13 activists who called for the protest, said the group resorted to the strike after the government failed to respond to letters sent to influential officials asking them to release the reformists, improve prison conditions and reform the legal system.
"We used all legal means to make our voice heard but we were ignored," said al-Qahtani, a college professor. "That's why we don't fear any government retribution."
The jailed reformists include Matrook al-Faleh, a human rights activist who was detained in May for advocating constitutional reform, and 10 other activists jailed in Jiddah in 2007.
The 13 men posted a statement on the social networking site Facebook to announce the strike and urge other Saudis to participate. Fifty-two people have so far signed up to join the 13 activists. They include writers, lawyers and college students.
Al-Qahtani said the participants chose to stay at home because "we don't want a confrontation with security forces."
"Freedom of expression and the freedom to gather are both banned," he said. "We don't want to have problems with security forces that would not lead to any results."
Al-Qahtani said there was no government reaction so far to their call. Government officials could not be reached Thursday, the start of the weekend in Saudi Arabia.
The kingdom's legal system, in which judges have wide discretion in punishing criminals, rules of evidence are shaky and sometimes no defense lawyers are present, has been widely criticized by human rights groups.
There have been reports of prisoners going on hunger strikes in Saudi jails but none about a public hunger strike like the one that began Thursday.
In December 2004, police responded with force to a dissident's call for demonstrations against the royal family.