BNP leader Ruhul Kabir Rizvi came up with the figure at a media conference in Dhaka on Monday.
"We are yet to gather information from some districts. The number of arrests will rise once we get it," he said.
Police launched the drive on Friday as there was no sign of an end to the rising spate of killings in recent times by suspected Islamist militants. The latest victims include a police officer's wife.
Out of the 8,192 arrested in the first three days of the drive, 119 are suspected militants, police said.
The others are accused in cases started over arms and narcotics, police said but did not reveal anything about their involvement in politics.
BNP Senior Joint Secretary General Rizvi said police arrested 'a number of innocent people' during the drive after their 'failure to identify the real militants'.
"The arrests aim not to stop the series of killings, but to prevent democratic movement.”
Rizvi also said the 'civil society members, including National Human Rights Commission Chairman Mizanur Rahman', have protested against the crackdown.
"The government is brazenly violating the High Court order not to arrest anyone without warrant," he added.