Ryan's action, just two days before he leaves office, and a day after he pardoned four other death row inmates, drew immediate angry reaction from prosecutors, the incoming governor and relatives of some of the victims.
CBS News legal consultant Andrew Cohen says, "This is probably the single most important event in the history of the death penalty since the mid-1970s, when the Supreme Court first outlawed and then reinstated executions nationwide. No governor, as near as I can tell, has ever commuted the death sentences of this many people, never mind doing it at the same time."
Ryan said he sympathized with the families of the men, women and children who had been murdered, but he felt he had to act.
"I am not prepared to take the risk that we may execute an innocent person," he wrote in an overnight letter to the victims' families warning them of his plans.
With death row inmates he had recently pardoned sitting in the audience as he spoke Saturday, Ryan framed the death penalty issue as "one of the great civil rights struggles of our time."
"Our capital system is haunted by the demon of error - error in determining guilt, and error in determining who among the guilty deserves to die," Ryan said. "What effect was race having? What effect was poverty having?
"Because of all these reasons, today I am commuting the sentences of all death row inmates."
Ryan had halted all executions in the state nearly three years earlier after courts found that 13 Illinois death row inmates had been wrongly convicted since capital punishment resumed in 1977 - a period when 12 other inmates were executed.
He said studies conducted since that moratorium was issued had only raised more questions about the how the death penalty was imposed. He cited problems with trials, sentencing, the appeals process and the state's "spectacular failure" to reform the system.
"Because the Illinois death penalty system is arbitrary and capricious - and therefore immoral - I no longer shall tinker with the machinery of death," he said.
Other governors have issued similar moratoriums and commutations, but nothing on the scale of what Ryan has done.
"The only other thing that would match what he's done is in 1972 the U.S. Supreme Court overturned the death penalty and 600 death sentences were reduced to life with that decision," said Richard Dieter, executive director of the Death Penalty Information Center.
The most recent blanket clemency came in 1986 when the governor of New Mexico commuted the death sentences of the state's five death row inmates.
Maryland Gov. Parris Glendening, who last year issued the country's only other moratorium on state executions, has no plans to pardon or commute the sentences of any death row inmate before leaving office Wednesday, spokesman Chuck Porcari said.
Ryan chose Northwestern University - where journalism students investigating Illinois death row cases helped exonerate some inmates - to publicly announce that he was commuting all Illinois death sentences.
Corrections Department spokesman Sergio Molina said Ryan had signed commutation orders for 167 people - 156 on death row and other in jails awaiting hearings or sentencing for other crimes.
Within a week the department will start moving prisoners out of the state's two "condemned units" and into the general population of maximum-security prisons, Molina said.
All but three of those inmates now face life in prison without the possibility of parole, Ryan said. The three will get shorter sentences and could eventually be released from prison, though none will be out immediately.
Vern Fueling, whose son William was shot and killed in 1985 by a man now on death row, was outraged that the killer would be allowed to live.
"My son is in the ground for 17 years and justice is not done," Fueling said. "This is like a mockery."
Incoming Gov. Rod Blagojevich, a Democrat, also criticized Ryan's action, calling blanket clemency "a big mistake." Each case should be reviewed individually, Blagojevich said. "You're talking about people who've committed murder."
Ryan on Friday went a step farther in four other death row cases, issuing pardons for four men he said had been tortured by police into making false confessions.
Cohen says it's highly likely the four will file unlawful imprisonment lawsuits against the state.
A few hours later, Aaron Patterson, 38, walked out of prison a free man and ate his first steak dinner in 17 years, while Madison Hobley and Leroy Orange spent time with their families.
Stanley Howard, 40, the fourth man pardoned Friday, remained in prison. He had also been convicted of a separate crime for which he was still serving time. All four had been convicted in murders.
"It's a dream come true, finally. Thank God that this day has finally come," Hobley, 42, said Friday as he left the Pontiac Correctional Center.
Orange, 52, walked out of Cook County Jail looking a bit dazed with his two daughters by his side.
"Thank you with all my heart and please do something for the remaining group on death row," he said, addressing Ryan.
The Republican governor announced the pardons Friday at DePaul University in the first of two speeches capping his three-year campaign to reform the state's capital punishment system.
Patterson's mother, Jo Ann, said she was overwhelmed when she heard the news.
"I don't believe in miracles but this is a miracle," she said.
Reaction to the pardons from death penalty supporters was swift.
Cook County State's Attorney Dick Devine said the future of the four men should have been decided by the courts. His office is trying determine if the pardons could be challenged, but Devine said the clemency powers for an Illinois governor are among the broadest in the country.
"Instead, they were ripped away from (the courts) by a man who is a pharmacist by training and a politician by trade," he said. "Yes, the system is broken, and the governor broke it today."
Cohen calls Ryan's actions "irrevocable."
Ollie Dodds, whose 34-year-old daughter, Johnnie Dodds, died in an apartment fire that Hobley was convicted of setting, said she was saddened by Ryan's decision.
"I don't know how he could do it. It's a hurting thing to hear him say something like that," she said, adding that she still believes Hobley is responsible.
"He doesn't deserve to be out there."