The first batch of 1,660 inmates is being released at 1.30pm today from Klong Prem Central Prison, director-general Nathee Chitsawang said.

Most of the pardoned convicts had committed petty crimes with less than a year to serve, or are ill or old.

Other prisoners have had their sentences commuted. Those on death row would be spared and spend their life behind bars, while lifers would receive a 40-year term.

Well-behaved inmates would have their penalties shortened by one-half for an "excellent" record, one-third for "very good", one-fourth for "good" and one-fifth for "moderately good".

Two infamous murderers, former medical student Serm Sakhonrat and former doctor Sorachat "Ham" Sirichote, were among those to receive a one-fourth reduction.

Those sentenced to over eight years in prison or handed life or death sentences before the 2004 royal decree on royal pardons went into effect did not qualify for clemency.

Among those not considered for a pardon are gynaecologist Wisut Boonkasemsanti, Major Chalermchai Matchaklam and Lt-General Chalor Kerdthet because their cases were still under appeal.