DUBAI | Sun May 6, 2012 6:21am EDT

DUBAI (Reuters) - Bahrain has arrested a prominent human rights activist and critic of the country's ruling family, the ministry of interior and an activist said on Sunday, as the authorities cracked down on pro-democracy protesters.

Police arrested Nabeel Rajab, head of the Bahrain Centre for Human Rights (BCHR), when he returned from Beirut on Saturday evening, Sayed Yousif Almuhafda, a member of the BCHR told Reuters.

"The police arrested him near the plane's door. They said they had an arrest warrant from the public prosecutions office ... He was allowed to call his family after the arrest yesterday but they could not see him," Almuhafda said by telephone from Manama.

"We don't know what the new charges are yet," he said, adding that Rajab is already facing old charges of calling for and taking part in unauthorized protests.

In an online statement, Bahrain's Ministry of Interior confirmed Rajab's arrest "for committing a number of crimes punishable by law". It gave no more details.

Bahrain, which is ruled by a Sunni Muslim monarchy and hosts the U.S. Fifth Fleet, has been in turmoil since activists mainly from the majority Shi'ite community began protests in February 2011 after successful popular revolts in Egypt and Tunisia.

The authorities tried to crush the uprising for democratic reforms with martial law and by bringing in Saudi troops, accusing activists of cooperating with Shi'ite Iran to change the system of government.

The ferocity of the crackdown stunned the Shi'ite community and more than a year later, unrest persists with weekly mass rallies by opposition parties and clashes between youth activists and riot police.

On Saturday, an improvised bomb wounded four Bahraini policemen as police clashed again with protesters demanding the release of Abdulhadi al-Khawaja, a jailed rights activist on hunger strike.

Rajab had been previously detained during a crackdown on Shi'ite protesters last year. Before this he had spoken to media about the crackdown and the BCHR had put out statements accusing Bahraini forces and their Saudi and Emirati allies of "massacres".

(Reporting by Rania El Gamal; editing by David Stamp)

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