Al Wefaq said it considers its leader, Ali Salman, to be under arrest hours after he was called in for questioning in what it alleged was a "series of illegal measures" by authorities.
Bahraini officials did not issue any comment on Mr Salman or immediately respond to questions.
Al Wefaq has been a major driver in Arab Spring-inspired protests that began in February 2011.
More than 65 people have been killed since the unrest began against the Sunni-led monarchy.
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