A special Bangladeshi court has sentenced a businessman to three years in jail on charges of corruption.
Mr Mamun is linked to Khaleda Zia's family
Giasuddin Al Mamun is reported to be close to Tarique Rahman, son of former prime minister Khaleda Zia.
Mr Mamun, managing director of a TV channel, was convicted for his failure to explain the source of his wealth.
Scores of top politicians and businessmen have been detained in Bangladesh since a state of emergency was declared in January.
The country's military-backed caretaker administration also suspended parliamentary elections that were scheduled for January.
It says it will hold the polls by late 2008, giving it time to deal with Bangladesh's endemic corruption.
Mr Mamum is one of the country's most controversial and powerful businessmen, the BBC's John Sudworth in Dhaka says.
Mr Mamum stood silently in the dock as he was convicted of failing to submit a statement explaining how he had amassed wealth that did not match his legally declared income.
Mr Mamun's lawyer, Fakhrul Islam, says he will appeal against the conviction.
The military-backed government says it wants an end to corruption
Police raided Mr Mamun's home in capital Dhaka's posh Gulshan area and arrested him in March. They accused him of possessing illegal firearms.
Later, Mr Mamun was named as a co-accused with Ms Zia's son, Tarique Rahman, in a case involving alleged extortion of $147,000 from a businessman.
Mr Rahman has also been in detention since March.
At least 60 members of parliament and former ministers face questioning under the state of emergency.
Mr Mamun is the fourth person to be convicted by the anti-corruption court.