A local BNP leader died at Kushtia General Hospital under custody on Thursday.
Resident physician of the hospital Dr Taposh Kumar Sarkar said MA Shamim Arju, joint general secretary of district unit BNP, died 10 minutes after he was taken to the emergency unit of the hospital from the jail.
Jail super of Kushtia Jail Zaker Hossain said Arju was sent to the hospital as he suddenly collapsed on the ground in the afternoon.
Local BNP sources said police arrested 10 BNP leaders and activists, including Arju, on March 26 while they were returning after paying tributes to the martyrs marking the Independence Day.
Later, they were sent to jail in a violence case.