(Reuters) - In 13 countries around the world, all of them Muslim, people who openly espouse atheism or reject the official state religion of Islam face execution under the law, according to a detailed study issued on Tuesday.

And beyond the Islamic nations, even some of the West's apparently most democratic governments at best discriminate against citizens who have no belief in a god and at worst can jail them for offenses dubbed blasphemy, it said.

The study, The Freethought Report 2013, was issued by the International Humanist and Ethical Union (IHEU), a global body uniting atheists, agnostics and other religious skeptics, to mark United Nations' Human Rights Day on Tuesday.

"This report shows that the overwhelming majority of countries fail to respect the rights of atheists and freethinkers although they have signed U.N agreements to treat all citizens equally," said IHEU President Sonja Eggerickx.

The study covered all 192 member states in the world body and involved lawyers and human rights experts looking at statute books, court records and media accounts to establish the global situation.

A first survey of 60 countries last year showed just seven where death, often by public beheading, is the punishment for either blasphemy or apostasy - renouncing belief or switching to another religion which is also protected under U.N. accords.

But this year's more comprehensive study showed six more, bringing the full list to Afghanistan, Iran, Malaysia, Maldives, Mauritania, Nigeria, Pakistan, Qatar, Saudi Arabia, Somalia, Sudan, United Arab Emirates and Yemen.

In others, like India in a recent case involving a leading critic of religion, humanists say police are often reluctant or unwilling to investigate murders of atheists carried out by religious fundamentalists.

Across the world, the report said, "there are laws that deny atheists' right to exist, revoke their citizenship, restrict their right to marry, obstruct their access to public education, prevent them working for the state...."

Criticism of religious faith or even academic study of the origins of religions is frequently treated as a crime and can be equated to the capital offense of blasphemy, it asserted.

EU STATES OFFEND

The IHEU, which has member bodies in some 50 countries and supporters in many more where such organizations are banned, said there was systematic or severe discrimination against atheists across the 27-nation European Union.

The situation was severe in Austria, Denmark, Germany, Greece, Hungary, Malta and Poland where blasphemy laws allow for jail sentences up to three years on charges of offending a religion or believers.

In these and all other EU countries, with the exception of the Netherlands and Belgium which the report classed as "free and equal," there was systemic discrimination across society favoring religions and religious believers.

In the United States, it said, although the situation was "mostly satisfactory" in terms of legal respect for atheists' rights, there were a range of laws and practices "that equate being religious with being American."

In Latin America and the Caribbean, atheists faced systemic discrimination in most countries except Brazil, where the situation was "mostly satisfactory," and Jamaica and Uruguay which the report judged as "free and equal."

Across Africa, atheists faced severe or systemic violations of their rights to freedom of conscience but also grave violations in several countries, including Egypt, Libya and Morocco, and nominally Christian Zimbabwe and Eritrea.

(Reported by Robert Evans; Editing by Cynthia Osterman)