Between 600 and 700 demonstrators were protesting peacefully outside the prison's East Gate as of 2 p.m., according to the Marin County Sheriff's Office.
The protest began at noon and was scheduled to continue until 3 p.m. A prison spokesman said San Quentin is on lockdown and that prisoners are being kept in their cells.
Organizer Barbara Becnel said the protesters are calling for a number of reforms, including the elimination of solitary confinement, the death penalty and California's "three strikes" law, and an end to the practice of trying some juveniles as adults.
The crowd included activists Sara Shourd, Shane Bauer and Josh Fattal, who served time in an Iranian prison after being arrested by Iranian authorities while hiking along the Iraq border in July 2009.
Shourd was freed in September 2010, and Bauer and Fattal were released about a year later.
Speaking at today's rally, Shourd said, "The prison officials say they need more time to end solitary confinement ... I say we need to keep the pressure on and we need to end solitary confinement now."
Bectel said the protesters have already achieved one of their goals: bringing the Occupy movement and the prisoners' rights movement together.
Occupy Oakland's website encourages its members to participate and lists carpool information for those who planned to attend.
She said the San Quentin rally is one of 15 taking place today at prisons throughout the country -- including in New York, Baltimore and Boston -- as part of National Occupy Day in Support of Prisoners.
The groups are seeking change at both the state and federal levels, and are planning an "Occupy the Justice Department" action in Washington, D.C., on April 24.
Authorities shut down the Interstate Highway 580 on- and off-ramps at East Francisco Boulevard during the protest.