Mainstream Bangladeshi newspapers carried the report on the alleged "plot" a day after Hefazat-e-Islam group's detained leader Junaid Babunagari gave a confessional statement before a magistrate after two weeks of police interrogation.
The reports quoting unidentified officials familiar with the statement said Babunagari disassociated himself from the plot but said other leaders of the radical group were sponsored by the opposition Khaleda Zia-led BNP and Jamaat to stage violent protests against the Awami League government on May 5 and 6.
He said the 18-party opposition alliance promised to provide all sorts of assistance - money, food and water as the Hefazat enforced a Dhaka siege blocking the capital's entry points from rest of the country on May 5.
On the same day, the group had staged a large rally in the heart of the city demanding enactment of an anti-blasphemy law with tougher punishment for humiliating Islam and the Prophet.
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Officials said 21 people were killed while unofficial figures put the toll as high as 28 during the violence in Dhaka and its outskirts and in south-eastern port city of Chittagong, the stronghold of Hefazat.
The 62-year-old Babunagari was arrested from the capital hours after the group's elderly top leader Allama Ahmed Shafi was expelled from the city.
Babunagari said he came to know from the group's volunteers that apart from some Hefazat workers, BNP and Jamaat activists were on the rampage and indulging in looting in many areas.