The official announcement of the amnesty was made Monday and by Tuesday prisoners were filing out of prisons, with as many as 15,000 people expected to be granted freedom.
The government also announced that "death sentences are commuted to life sentences" and "other prison terms are commuted by one year," a report in the government newspaper New Light of Myanmar said.
The decree, signed by Thein Sein, former junta prime minister and now Myanmar's civilian president, said Myanmar was a "peaceful, modern and developed discipline-flourishing democratic nation."
The amnesty was to "turn prisoners into citizens who will in one way or another contribute toward the process of building a new nation."
It was granted by the president out of "humanitarian" concern and "out of consideration for the families of the inmates," the announcement said.
But Human Rights Watch reacted strongly, saying very few of the country's 2,200 political prisoners were among those being released. The one-year reduction in sentences for political prisoners serving 65 years was "a sick joke."
The BBC's Burmese radio service said it estimated around 30 political prisoners will be among those freed.
Human Rights Watch said the most prominent political prisoners are likely to remain behind bars.
These include Zargana, Myanmar's most famous comedian. He is serving a 35-year sentence for criticizing the government's slow response to help people after the devastating Cyclone Nargis in May 2008 in which around 138,000 are believed to have died.
U Gambira, a 30-year-old monk who helped lead the August-September 2007 protests is serving a 63-year sentence and won't be released.
Nay Phone Latt, a 30-year-old blogger on the 2007 protests was sentenced to 12 years in prison.
Su Su Nway, 40 and a female labor rights activist, is serving an 8 1/2-year sentence for raising a banner at the hotel of a visiting U.N. special envoy in 2007. The banner was critical of the government.
The amnesty announcement came after the visit by the U.N. secretary-general's Myanmar envoy, Vijay Nambiar, last week. He said the government, which took office officially at the end of March, needed to do more to address the concerns of all segments of society and release all political prisoners.
"Only then can there be greater confidence that the efforts undertaken will indeed serve to meet the long-standing needs and aspirations of the people of Myanmar," he said in a statement. "There is no time to waste if Myanmar is to move forward."
Human Rights Watch said ordinary petty criminals welcome the amnesty.
"But for the 2,100 political prisoners unjustly serving sentences of up to 65 years, the one-year reduction is a sick joke," Elaine Pearson, deputy Asia director at Human Rights Watch, said. "This is a pathetic response to international calls for the immediate release of all political prisoners."
Critics, including Human Rights Watch, claim the amnesty is a cynical move by the government to garner international recognition after the general election in November. Most Western governments consider the poll fraudulent and not inclusive of many active pro democracy parties.
Sein, a former general, is a long-standing ally of former junta leader Senior Gen. Than Shwe, 77, who had ruled Myanmar, formerly called Burma, since 1992.
Sein led the Union Solidarity and Development Party in the general election. It comprised mainly retired military officers who resigned their posts to join the party and run as civilians.
Not unexpectedly, the USDP won a huge majority in the general elections.
Also, one-quarter of seats in Parliament were reserved for military appointments.
Absent from Parliament is the winner of the last national elections in 1990, the 1991 Nobel Peace Prize Laureate Aung San Suu Kyi. Her National League for Democracy party, which won the 1990 contest, didn't register as a political party because Suu Kyi remained under house arrest.
She has since been released.