Mr Demirel sent the bill back to parliament asking MPs to make it compatible with "justice, rules of equality and public conscience". Parliament had passed the bill on Saturday as the country was still reeling from the 17 August earthquake, then adjourned for a vacation.
Backers initially meant the amnesty as a solution to crowding in Turkey's prisons, where riots, hunger strikes and hostage-takings are common. But the proposed release of thousands of murderers, gang members and corrupt officials, some with links to the ruling coalition, outraged the public. Opponents said it would forgive crimes against citizens, but not against the state.
The right-wing hitmen who shot Akin Birdal, a human-rights activist, would be freed, for example; Mr Birdal himself - serving a nine-month term for opposing the government war with Kurdish rebels - would not.
The bill applies to most criminals convicted before 23 April who received sentences of up to 12 years. All juveniles would be freed unconditionally. The law also provides lighter prison terms for many other inmates, including murderers.
Even the Prime Minister, Bulent Ecevit, who initiated the bill, distanced himself from it in its amended form.