(Corrects word to billion from million in final paragraph)

BEIRUT, Feb 27 (Reuters) - One of the most respected clerics in Shi'ite Islam called on Wednesday for action against a school of thought used by militants to justify killing other Muslims. Lebanon's Grand Ayatollah Mohammed Hussein Fadlallah described an attack on Shi'ite pilgrims in Iraq on Sunday as "the pinnacle of barbarism". The bombing was blamed on the Sunni Islamist network al Qaeda.

In unusually strong language, Fadlallah described the perpetrators as "murderous animals", who "regard as permissible (spilling) the blood of Muslims who embrace another doctrine, or believe in alternative political views".

Fadlallah is based in Lebanon and has Shi'ite followers across the Muslim world.

In a statement he said the phenomenon of Muslims charging others with non-belief, known as "takfir" in Arabic, was "one of the most dangerous issues" faced by the Muslim world.

A person accused of non-belief is regarded as a legitimate target by some militant Muslims.

It had "produced a murderous, aggressive barbarism dressed up as holiness and religion and stemming from ignorant and fanatical religious edicts", said Fadlallah, one of the Muslim world's few Grand Ayatollahs.

Fadlallah hit out at what he described as "silence in the Islamic world over these criminal operations which annihilate children, women and the elderly, from Afghanistan to Pakistan, to Iraq".

"We want the Islamic nation to confront this aggressive condition, politically and culturally and at all levels."

Sunday's attack in Iraq targeted pilgrims making their way to Kerbala for a rite known as Arbain, which falls 40 days after the annual commemoration of the killing of Imam Hussein -- the grandson of the Prophet Mohammad.

Hussein's death in battle in 680 was a defining moment in the history of Shi'ite Islam.

Shi'ites, who make up a minority of the world's 1.3 billion Muslims, believe leadership of the Muslims should have passed to Hussein's father, Imam Ali, and his line after the Prophet's death. (Writing by Tom Perry; Editing by Robert Woodward)

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