The rapid implementation of strict Islamic laws in northern Nigeria as a means of gaining political advantage has intensified human rights abuses, Human Rights Watch said in a report made public on Tuesday.
Nigeria, Africa's leading oil-producer, has 36 states of which 12 predominantly Muslim states have declared Sharia law since 2000.
The effect has been to polarise Nigeria's population of 130 million, roughly divided by Christians and Muslims, as tribal and religious communities compete for wealth and political power, said the New York-based group.
Lack of legal representation, acceptance of testimony extracted under torture and inadequate training of Sharia court judges are all failings characterizing Nigeria's Sharia law, which has been speedily implemented for political purposes, the report said.
While such failings are not specific to Sharia courts, they often lead to disproportionate human rights violations given the severity of some of the sentences under Sharia, such as amputations, floggings and even executions by stoning.
Women face heightened discrimination as the standards of evidence for Sharia crimes such as adultery differs for men and women.
"The use of religion as a political tool has, if anything, increased in Nigeria over the last few years ... Failure to respect due process has led to serious but preventable violations of human rights," said the report.
While some sentences have been appealed at the state level and few punishments have been enacted, there has been no attempt by the federal government to redress the lower Sharia courts responsible for handing out the punishments, it said.
Analysts say this is because Nigeria's president Olusegun Obasanjo, a born again Christian from the south of the country, is eager to cut deals across ethnic and religious lines to keep his ruling People's Democratic Party (PDP) in power.
Meanwhile, a surge in religious conflict has accompanied the country's return to civilian rule since 1999, when Obasanjo's election ended 15 years' rule by the military, then dominated by Muslim generals from the country's north.
Many Nigerians say that since 1999 the country has seen even greater levels of poverty, crime and violence. At least 11,000 have been killed in sectarian violence.