Vice-president Nabeel Rajab said on Thursday that the original law, passed in 2001, was meant to protect those who took part in a genuine struggle for political reform in the 1970s, '80s and '90s.
But he says it has been invoked to dismiss over a dozen cases brought by the Bahrain Centre for Human Rights (BCHR) against government security officials, who killed over 40 detainees and tortured many more during the last three decades.
"The original decree  was amended in 2002 by royal decree 56, which was an obvious attempt to shield officials, who tortured and detained people during the years of unrest," he said.
Truth and reconciliation
"Any country that goes from a dictatorship to a reformed system of government should not cover up the crimes of the past. We want a South African-style truth and reconciliation commission", Rajab said.
"It's the only way we can move on."
He added that the UN had also called on Bahrain to amend decree 56, saying no amnesty should be used to protect those in the government who committed crimes against humanity.
Expressing limited optimism for the BCHR campaign, Rajab concluded that its "drive to get 56 abolished enjoys wide-spread support among the Sunni and Shia sides of the community, because both suffered".
Bahrain's Shia are believed to form a slight majority of the country's 400,000 citizens. The ruling family belongs to the Sunni branch of Islam.
Bahraini government media spokesman Hussein Jassim told Aljazeera.net that he was sure that "not everyone" in his country held this view, but refused to give a public statement denying the accusations.
Bahrainis marching on 17 June
2005 to demand reforms
However, London-based Bahrain analyst Dr Omar Al-Hassan disagreed with the BCHR view and said decree 56 had played a positive step.
"It must be recognised that instances of officials abusing their position during the unrest, which affected all parties, were exceptions rather than the rule."
Al-Hassan, director of the Gulf Centre for Strategic Studies, also invoked South Africa's Truth and Reconciliation Commission, saying King Hamad's decree had granted amnesty to all parties for "a united and prosperous future rather than dwelling on the past".
"The king has, in my opinion, taken necessary steps to benefit all members of society and to offer Bahrain a better and more prosperous future", he added.
But critics, who demonstrated in Manama on 17 June, accused the government of not doing enough, pointing to political issues such as the appointment, rather than election, of the upper house of parliament.
And in a human-rights report, published on Wednesday, another Bahraini watchdog called for an independent judiciary and urged authorities to amend laws governing societies, clubs and the media.
The Bahrain Human Rights Society also called for the establishment of political parties, a solution to the burgeoning unemployment problem, protection for expatriate labour, and a major push for the release of six Bahrainis detained at US-run Guantanamo Bay prison camp.