By Mirwais Harooni

KABUL (Reuters) - Human rights groups reacted with dismay on Wednesday to Afghanistan's resumption of executions after a virtual four-year moratorium on a penalty that characterized the austere rule of the former Taliban regime.

Eight men found guilty of "crimes against the people, especially women and children" were hanged on Tuesday and eight more were to meet the same fate either on Wednesday or Thursday, according to a government official, alarming activists who said the return of executions in a country with a weak judiciary was a blight on gains that had been made.

"The death penalty is an act of cruelty that should never be used," said Brad Adams, Asia director at the New York-based Human Rights Watch.

"Its use in Afghanistan, where a fair trial is typically out of the question, is even more horrific."

Those executed included a man who strangled to death a mother and her two infant children and men found guilty of kidnapping, raping and murdering young boys and girls.

Officials said that in accordance with the law, President Hamid Karzai himself signed off on the executions. There have been only two executions in Afghanistan in the past four years.

"The eight hangings in a single day are a terrible step backwards for Afghanistan. President Hamid Karzai should stop future executions and commit to a formal moratorium," Adams said.

Rafi Ferdous, a spokesman for Afghanistan's Council of Ministers Secretariat, defended the hangings.

"By applying this (penalty), the rule of law is implemented. This is a lesson to be learned," Ferdous said in a statement.

Executions were commonplace during the Taliban's strict 1996-2001 rule of Afghanistan, when adulterers and murderers were shot dead in public. Such executions still take place in some areas under the influence of the Taliban.

But judicially sanctioned executions have been rare since the Taliban government fell, with unofficial moratoriums between 2001 and 2004 and from 2008 until recently.

The death by firing squad of 15 prisoners in Kabul in 2007 and several more executions in 2008 sparked international condemnation and until Tuesday, only two people had been put to death in the past four years.

Hussain Ali Moin, an investigator with Afghanistan's Independent Human Right Commission, said that although the death penalty was legal, there were concerns about whether those executed had received fair trials.

"The death penalty is an irreversible punishment and a trial must be fair and all principles for a fair trial must be respected," he said. "We haven't had full access to these cases... Still we cannot say if their trials were fair."

(Additional reporting by Miriam Arghandiwal; Writing by Martin Petty; Editing by Nick Macfie)