The fire started around 1:45pm, the Fire Service said
The blasts took place around 9:15pm within 150 yards of the office, where the BNP chief has been living since Jan 3.
About 50 police personnel have been posted around the office, which is experiencing a power cut since the early hours of Saturday.
Meanwhile, four pro-BNP lawyers including its women's front leader Khaleda Yasmin were nabbed by police in front of the office.